We are caregivers for my 95 year old mother in law. She has what is considered moderate dementia. Sleeps about 17 hours day. Is that common?

Yes, as others have pointed out, it's probably time for Hospice. Be aware that Hospice is no longer just for people for whom death is imminent. Anyone not improving, or in a state of decline can qualify.
Pain is horrible and it's exhausting and daunting to deal with. No one should have to endure it if there is an option. Hospice offers an option.
Best of luck to you all.

At 95 she may be entering end of life. I suggest that you get a referral for hospice evaluation. At end of life LOs sleep more and eat less. My father went through that at 91. While suffering mixed dementia he developed heart problems and opted for hospice rather than continued treatments and rehab.

His dementia advanced, he slept more and ate less. Hospice made sure he was as comfortable as he could be though he never complained of any physical pain.

Hospice can provide meds to keep her pain levels at a more manageable level.

I wish you and your family the best.

Sleeping a lot is a sign of decline. It is one of the things that I kept track of when my Husband was on Hospice. They do take increased sleeping a an indication of decline when they are recertifying a patient.
With the pain level, the increased sleeping and decreased food intake you might want to contact Hospice.
As she declines you will need more help and equipment to care for her. You will get the supplies, equipment and help from Hospice.

It sounds like perhaps it may be time to get hospice involved with your MIL's care. They can better control her pain, and keep her comfortable, which is something we all deserve as we age.
My husband in the last 6 months of his life was sleeping anywhere from 18-20 hours a day. He was under hospice care in our home for the last 22 months of his life, and suffered from a lot of pain as well.
The nice thing with hospice is that they will supply a hospital bed for her if needed, along with any other needed equipment, supplies and medications, all covered 100% under your MIL's Medicare. They also will have a nurse come once a week to check on her and her vitals, and will have aides come about twice a week to bathe her.
So you may want to have her doctor order hospice for her, as they will get started right away.

It's common for dementia patients to sleep a lot as the condition deteriorates and they approach the later stages. It sounds your MIL needs a hospice evaluation asap, for the pain situation alone, which they can help her with. Over the counter pain killers probably don't help her at all with the level of pain she's experiencing, and that's a terrible thing to endure. If she's barely eating, that's another sign she may be approaching end of life and again, a hospice evaluation can be ordered by her primary care doctor.

Your profile says she's living in Independent Living? Hopefully she's not living alone with you as her part time caregiver, b/c with dementia & all this pain, she really requires 24/7 care now

Here is a link to a chart showing the stages and symptoms of dementia and what to expect:

https://www.dementiacarecentral.com/aboutdementia/facts/stages/

Wishing you the best of luck with a difficult situation.