Three years is a long time for bulbar onset of ALS, from what I understand.
Do you know what his lung capacity is at the moment? From what I have heard, anything below 35% is getting into a dangerous level where mechanical ventilation would be necessary.
I'm assuming he is using BiPap, but you didn't mention it...is he?
If he is sleeping through the night without waking with restlessness, he may still be doing pretty well breathing wise.
As far as "end of life" signs - sometimes you will notice irregular breathing (starts, stops, quick breaths, no breaths, ect....) this can go on for awhile - hours, days?
Also, for end of life (and this is not specific to ALS...it's end of life stuff for many illnesses/conditions) you may notice little urine output, not wanting food/drink, sleeping a lot. It's possible that they may run a slight fever - sweat, not sweat, hot then cold (indicating that their temperature regulation may be out of wack). Finger tips may look blueish in color (this could be the case for ALS patients sooner than other patients, because of the hypoxia issue - lack of oxygen). Heart rate may be slower or irregular.
The nursing staff won't give you a time table because honestly they don't know. I've worked hospice before and when patients looked really bad to me, all I would say is, "There is no way for me to tell, it could be hours or weeks , but it doesn't look good."
If the patient isn't drinking or obtaining fluids in some way, it probably would be about 7 days or less. Same if the kidneys shut down (no urine output).
I hope this helps in some way. Very sorry for your family.