How do I know when it's time for hospice? My mom has end stage COPD, she was recently on life support but is now on bipap machine again.

COPDers suffer a great deal. Each setback, they may say that they can take a few more hits. I look at COPD like a lobster set in cold water while it comes up to a boil, does not realize how much they suffer compared to other diseases. You notice your mom suffering but she is not ready. The only advice I can give is that maybe you can ask for a hospice consult and let the specialist go and talk to her. At this point, ethically, you cannot make the decision for her but you can ask for someone to speak to her about how worse she can get.

YOU do not make the decision for Hospice.
SHE does.
You are describing someone who is choosing heroic measures to live.
That is her right. If she is competent this isn't your choice.

If you mom chooses not to be put on a ventilator anymore, and not to seek lifesaving treatment, has accepted that she is end stage and is dying, and wishes no more than comfort care, that is when she interviews and enters Hospice with the order of her treating MD.
Discuss with HER her own wishes. Follow her wishes.

If your mother becomes unable to make her own decisions, and you believe her to be suffering needlessly without hope for a quality of life, then the doctor will ask you to make her decisions for her, in her best interests to the best of her ability.

YOU do not make the decision for Hospice.
SHE does.
You are describing someone who is choosing heroic measures to live.
That is her right. If she is competent this isn't your choice.

If you mom chooses not to be put on a ventilator anymore, and not to seek lifesaving treatment, has accepted that she is end stage and is dying, and wishes no more than comfort care, that is when she interviews and enters Hospice with the order of her treating MD.
Discuss with HER her own wishes. Follow her wishes.
This is her choice, not yours.

If your mother becomes unable to make her own decisions, and you believe her to be suffering needlessly without hope for a quality of life, then the doctor will ask you to make her decisions for her, in her best interests to the best of her ability.

Sounds like your mom still wants all life saving measures to be taken to keep her alive at this point, so hospice would NOT be a good option for her now, as they will remove all life saving measures.
There will come a point when your mom says enough is enough and that is when hospice should be brought on board.