Sorry, I can't remember how long exactly. I was on 2 or 3 T-shirts a night too. I think after the first couple of weeks it got a bit better, but it was certainly several weeks if not a couple of months before I felt more normal. I still get hotter at night than I used to, and ds is nearly 16 months . I hope yours improves sooner.
Lollypop, I hope you don't still need a lot of water to drink overnight ... it can be a symptom of diabetes. My dh has diabetes and severe thirst at night was the symptom which first prompted him to go to the doctor.