Near the end, but how to judge how long? Six days, six weeks, six years it's so hard!

There are medications that will help with the drooling. When my Husband was on Hospice they prescribed Atropine. (It is prescribed for eyes but it helps dry secretions in "off label use") There are other medications that can be prescribed for drying secretions as well.
There are typical signs that indicate that death is close.
Discoloration to the feet and legs called Mottling.
Sometimes a person will develop a pressure sore called a Kennedy Ulcer, typically near the tailbone area, death is usually within 24 hours but this does not always occur if it does death is rapid.
A change in breathing called Cheynes Stokes breathing. Secretions can build up in the back of the throat tun her on her side to clear do not use a swab as that can cause a gag reflex.

There is a wonderful pamphlet that you can read on line called "Crossing the Creek"

If she is not on Hospice you should call. The Hospice I volunteer with is still sending staff into homes and they will help a lot with information and supplies. The help is not just for her but for you as well.
((hugs))

All these responses (helpful for sure) bring back the realization that living a total-non-life is NOT life at all. <And a terrible drain on the caregiver.> Has she ever revealed to you that she’s simply had enough.?? (My mother & both grandmothers did.) Is she suffering?? IF she has indicated so, IF your state permits and IF the religious beliefs of you both don’t contradict: could you consider aiding her departure?
This suggestion is mentioned in the interest of the wellbeing of you both. And, no, I did not resort to this.
Particularly because hospice now is marginal everywhere, you may not be able get the help you both deserve. My top concern would be to alleviate suffering. All good wishes to you and your LO; this is as tough as it gets.

Have you done a telemedicine appointment with her doctor and described to him or her what you’ve related here? If not, that should be done to hopefully help with getting more info on what to expect. Also, ask about hospice services as that my be a help for you if yoir wife isn’t already receiving that.
When my mother was nearing the end, the signs were that she stopped communicating with us, stopped accepting food, and slept far more, with the sleep being deeper than usual. Once this was the consistent pattern she passed away in three weeks.
Do you have help?

My mom made it 3 1/2 days once she couldn’t eat or drink. She was non responsive so eating and even water was impossible,
We had hospice as well, they were a great help, but weren’t very helpful with timelines, I think it’s just really different for each person. They used scopolamine patches at the end for the secretions, it really helped mom.
You might look into a pure wick system for urinary incontinence. It’s an external device that suctions away urine.

The basic answer to your question of "How long" before death, is that it is very hard to know. Some answers below have given indications, If she stops eating and drinking, that is a clear sign that death is near, possibly days or a week. This would not be painful for her, as she would slip away peacefully, unless there is some kind of intervention from the cancer.

The sleep pattern is challenging, but not surprising. There are clinical studies that suggest that people with dementia take more sleep in order to lessen the effects of the tangles and Tau deposits in the brain. The long sleeps do not stop the tangles and Tau deposits forming, but they seem to minimise their impacts.

My wife of 57 years, now 10 years into Alzheimer's has been in bed now at home for the past 22 months. In September 2018 we all thought she was dieing, but she rallied and and has still retained the ability to communicate to a considerable extent. A local hospice has given very helpful advice. What she eats and drinks is very important. We find that coconut water with Kafir works well, and aim for 1,000 ml per day. Food needs to be mashed up so that it does not require too much chewing. We have considerable help from local Home Instead CAREGivers, which gives me space to do other activities.

It is not easy, but if possible care at home is certainly the safest and best option at this time, when care homes and nursing homes are so threatened by Covid-19. However, you need to set up a life style that is viable for you as well as your wife. Small changes can make a big difference.

Prayers and hope for the future, which remains unknown.

This is so difficult. My mom passed away in February and I had many of the same questions you have now. Here comes some free advise. I was so tired that I called hospice and requested an evaluation. I knew that I needed more help. I asked for respite care. I found out that she could go into their hospice for 24 hour care. I realized that I could not deliver the level of care she needed. She would get 24 hour care and I would get a rest. It worked out that she got the care and I got the respite and she passed away. Although I knew the end was near, I was in no shape to have her die at home. She had received excellent care in my home for 6 years which I provided. When the time came, hospice really helped me. I will always donate time and money to their cause. I did check out several hospices and found a good one. Some are for profit and some are non profit. Work with your doctor and come up with a plan. Please do not try to do all of this by yourself. I tried but things worked out when I got help for both of us.

Sometimes people hang on because they have something to complete. We hear stories about folks who waited until all the family had visited before they were able to let go. That happened with both my father and my father-in-law. With my Dad, I had to tell him, even though he could not respond, that it was OK to go. He died that morning. People who die when their lives are done (that is certainly not true for all) are very fortunate, but sometimes they need help to understand that the time of completion is here. It may take some self reflection on your part, but being able to have that "talk" is sometimes the kindest and most loving thing we can do. Just adding my 2 cents.

I can't give you advice because you're in a really really tough situation. But I can pray for you--and I will!!!!!!!!!!!!!

Lots of good answers about how to care for her, but to keep steady, you need to care for you too. That's an important consideration. It's a tortuous waiting game. I experienced it with my MIL and my husband. I learned to do the simple things like breathe, go outside, and other things to keep my mind steady. And I found each day that I prayed about giving them the space to navigate this transition. That it's about making space for that person to find their way, because we really can'thelp them with that. Only support them from the side with love and understanding. It's like they're walking in a field or maze, gradually finding their way. And then each of them died when we caregivers were literally looking away from them for a few moments.

Tare care of yourself so you can continue to be that support for your wife. And recognize and honor the grief you must be feeling as well. I found talking to the hospice social worker was enormously helpful.

Have you talked to her doctor? You said "we have to change her" is she on hospice care now? If not, it may be time.