The rate of ALS progression is a key factor but at 20%, all of the things Laurie mentioned are really important. But the critical thing is usually whether you catch a simple cold or respiratory flu. That is very often the endpoint and isn't predictable so can't be used to estimate the time you have left. At 20% there is no room in the lungs for both breathing and congestion. Even BiPAP may not be enough. Aspiration pneumonia can happen from saliva even if you are not eating. Sorry to be so grim but that is the way it is.
Thanks for the answers. I know it all depends on a million different things but the Dr brought up hospice for the first time so I was on alert. His weight hasn’t changed, he still eats fine, but FVC has tanked, especially so over the last couple weeks it seems like.