Hello
@jzbross and welcome to Dementia Support Forum. There is a wealth of shared experience of dementia to be found here so I am glad you have found us.
I am sorry to read of your Mum's situation. and that she is now on end of life care. That normally means that she will be, as far as possible, be kept comfortable and pain free. It is likely that the GP will have prescribed anticipatory end of life medication, in the event that your Mum gets distressed with pain or the like. These meds would only be given by a trained medical person/community nurse or similar.
As far as food and drink goes, your Mum would be offered both, but if refused, that would be accepted.
My Mum had the same experience, and the care home were just great at the time. As for how long, that can vary hugely from person to person. there is no saying how long end of life care can be.
I am so sorry that you are having to go through this with your Mum. All I would say is spend as much time with your Mum as you want to, hold her hand, and tell her you love her (they say hearing is one of the last faculties to go). This is what I did with my Mum. Hope this helps and wishing you strength to get through this horrible time.
The following link may help too.
End of life care should help you to live as well as possible until you die, and to die with dignity. The people providing your care should ask you about your wishes and preferences and work with you to plan your care.
www.nhs.uk