As with most things having to do with our boats, "it all depends." I think that the temperature, wind, and humidity on the day you paint would be major factors in how long the paint takes to dry enough to put on a second coat. And that means the temperature and humidity of your hull as well as the ambient temperature outside. Having said all that, and while I agree that you'll hardly ever go wrong following the instructions on the can, my experience after painting a lot of boat bottoms over the years (mostly with ablative paints) tracks well with what Casey states. That is, I usually try to bottom paint my boat on day that is over 60-deg and relatively dry and sunny. By the time I've reached the other end of my boat, the end I began on is totally dry and could accept another coat of paint. I don't put a second coat on the whole hull, but rather only the waterline and leading edges of the keel and rudder and right under the bow. I've never had a problem with the paint in those areas not adhering well. Moreover, my marina -- perhaps like other marinas -- paints the jackstand pad spots for me on the day of launch if I leave them some paint and a chip brush. Interestingly, the boat is usually in the water not much more than a half hour after they paint those spots. I also have never had a problem with losing paint on those spots, which suggests the paint has fully dried in the half-hour before launch.
Finally, I can paint the bottom of my H28.5 with a roller in less than an hour. It normally takes me longer to get ready to paint, i.e., to tape off the waterline, stir up the paint, set out tarps, etc., than it does to actually paint the bottom....which I hope to be doing about one month from today!!!
