My mother is dying and her oxygen levels have been 78 all day. How long can she live at this level?

It was when my mothers oxygen hit 70%, that hospice told me she was "transitioning". They said to expect about a week- mom passed early the next morning.

I see that others here are saying longer - I honestly don't know that much about it. If it makes a difference - my mom had not been eating much for quite some time and not at all for a bit. Maybe that contributed to a fast exit.
I'm sorry your having to go through this - it's tough to watch someone linger.

Roxy, there is no way to know exactly. My father's bp, temp, and O2 went down very low and he lived for two more months. He was able to still walk a little at this time. If your mother is bed-bound and actively dying, then all you can do is wait. Letting go of life is not a simple thing for body and spirit to do. Your mother will know when it is time, so all that can be done is wait until it is time for her to leave.

A person can live many months with O2 sats in the 70's or even lower. There'll be a blue tinge to the skin and lessened mental acuity likely, but it doesn't preclude living.

Roxy, is your mother on hospice care? Those people are very good with questions like that. Helping you deal with this process is part of their role.

You will see O2 levels drop and heart rate increase, until the heart reaches a point where it can do no more.