Bed bound and stopped eating how long can you survive?

my mum has degenerative cerebellar ataxia and cerebellar atrophy, she is bed bound and can't move anything except her head slightly, sometimes she can respond, she has fluid on her lungs and has now stopped eating, she is only 64 and had every test under the sun but there is nothing that can be done, i was wondering how long can she live without food? days? weeks? thankyou Tracy