I'm so sorry op, I've done this three times in the last 15 months with parents (mine and dhs). This has been really sudden and you must all be in shock. This is the worst part of it, the not knowing when it will happen or for how long you will be there and the apprehension of waiting for them to die
The most recent was last month. There will usually be physical signs it is getting nearer. The nurses can usually tell you if death is approaching soon. If it isn't, make sure you are getting out a walk along to the cafe when it opens (if there is one) and for something to eat, for a bit of fresh air outside if no cafe.
We were also given use of a family room and each of the three of us went and had a nap during the night for an hour at a time. They had just put hospital bed mattress on the floor but that really helped, if you can ask about that?
Hopefully the palliative nurse will be in this morning and able to give you a better idea of timescales. Last month ours came in the last day and said itl be today and she was right. Then we noticed the physical changes which indicated it would be soon, called a nurse in and she said itl be in the next hour provably and it was 50 minutes later.
Are you in a private room with her? If not, insist on one. You might need to be forceful about this, we were. If you are and you live close and don't have any, can you get toiletries and clean clothes brought so you can use the shower to freshen up.
I hope that she is at peace soon, sending lots of love and strength for you to get through this difficult time.