Hi Packgirl: I did water therapy at 3 mos post-op but not before, because that's what my Dr insisted on. I would think that when you return, it will be when your Dr say's you can.
Did you go to a PT office for your water therapy or was it through someone else? If you can find a PT office that has a pool I think that would be better than say a city that runs a water aerobics class, since a PT knows more about back surgery rehab.
I went for 1 1/2 years after surgery, but that was before my insurance put a cap on PT.
When you return, just make sure you go at your own pace, and a good PT would know that's necessary. I know how strenuous those classes are, but what I did was start out in a 1 on 1 class with the PT, then slowly moved up to the group class when I was able, which was more difficult. At some point you might want to move to shallower water when you can handle the gravity, but start out in the deep water.