Poop, poor you. If it helps, I've been there and we all lived to tell the tale. When DS was 15 months old he was stupendously sick with a gastic bug: sick 15 times in 12 hours, bringing up bile, vomiting up every bit of fluid given to him, the lot. We phoned the doctor 3 times during the night and he was great - he didn't come round but would have done and talked us through it and phoned us back in the morning.
Basically he said to us that dehydration is fairly rare in babies of a reasonable size and health pre bug (I stress DS was 15 months and a bit of a whopper so can't answer for anyone else) - I think I assumed from the books that it would happen in a matter of minutes. I remember him asking if DS was listless or floppy - well its quite hard telling when they've just been sick for the 8th time and its the middle of the night.
Other advice - avoid milk and all milk products while they are still vomiting. Fluid is by far the most important: we were told to put very small amouts in a bottle or beaker and give it to them every 15 or 20 mins. We did this for about 4 hours. It was just awful how ds would cry when he wasn't allowed any more fluid, but you can get a surprisingly large amount down them in this way - and it stayed down. The doctor did say that even if they brought it all up, they would be still benefitting from the small bit that they did absorb. Dioralyte is good, disguised in whatever way you need to.
DS was very ill for about 36 hours, then very poorly for 2-3 days (lying on the sofa, no energy, etc etc). It took him about a week to be fully interested in food again. We started him back on breadsticks, rice cakes and fairly inert things.
When DD was "merely" sick 4 times in a night recently it was a breeze - we felt like real veterans. In a way, it was helpful our first experience being so bad as it meant the next times were much easier to deal with.
Look after yourself to the extent you can - the worry and the piles of sick-sodden bedding and clothes everywhere are quite demoralising (oh and I was 8 weeks pregnant when DS did all this, so dealing with huge amounts of vomit wasn't exactly welcome!)