Depends on what you mean by "get wet". I would personally avoid taking her swimming (too much exertion anyway, on the stomach muscles) or giving her a bath her incision is largely healed. She won't be needing any dirty water or soap in there to irritate things. So all going well, that's probably about a week to ten days, or so.
If you just meant getting wet because it's raining when she needs to go out to pee, then I don't think it's any particular cause for concern. If her belly gets a bit wet that way, I'd just dry her off gently once she's back indoors.