Your choice, I just put it in at the start and let it warm up/melt as the water is coming up to temp. Once the water starts to boil, I'm guessing within 20 ~ 30 minutes the wax will be totally melted.
You leave it in until the wax melts. Once melted, remove from the heat and let it cool back down (this will probably take a couple hours to be come a solid again). I personally just turn off the stove and let the whole thing cool down. Wax it highly flammable and the less moving of it the happier I am.
I'd probably start this process in the evening and let it cool overnight.
With that said, once you start this process, I would HIGHLY advise that you don't leave the pots unattended until you have removed the heat source at the end.