My wife and I have been doing Spinning @the Y to help get ready for the cycling season.
As I was there today I was wondering about how much water I should be drinking during a 50 minute class?
Thanks!

Enough to replace what you've lost to sweat. If you wanted to be really scientific about it, you'd weigh yourself before, not drink, and then weigh yourself after. Or if you were like me, you'd just drink when you got thirsty.

Drink a bottle if you like, or nothing. Cycling for 50 min isn't long enough for dehydration to affect your performance but you do need to replace the fluids eventually.

One 750 ml bottle of water should be ample.

I agree. It depends on various things. How hot is the room? Do they have fans? How hard do you work? Do you sweat a bunch? If the room is cool with fans and you do not work hard, you might only need a few sips. If you push really hard and the sweat is dripping off of you, well you might just need 2 bottles.

I used to take 2 500ml bottles but have now moved onto 1 750ml bottle

I loose 2lbs in my 50min spin class [+10 warm up]. I rarely drink until the end.