Hi Eric, 

i just finished varnishing a hull myself...so this is a good question.

i would generally say, wait at least 24 hours.  but the real answer is to ensure your varnish is hard enough not to be easily dented...

an easy way to tell is to push your finger nail into the surface and if you easily make a dent....you need to wait.  sometimes if your temperature is below 70 degrees or you put on thick coats of varnish this can take more than 24 hours to not be easily dentable.

that said the other thing i do when i flip the boat over is to have a really cushy/wide support for the boat.  i use old bath towels over my sawhorses just to ensure i am not putting too mush pressure on a small surface area.  varnish, like paint coatings, continue to harden over a number of days when the temperature is above the minimum application temperature....so i always treat the boat delicately when the paint or varnish is fresh.

i hope this helps.

h  

   I used a helmsman spar urethane and it dries very fast. I was able to do 3 coats a day; one early morning, one mid day and one evening. I used gloss for all the coats except for the final 2 on the deck I did with a semi gloss. 

I then left it a good 2 weeks to harden before putting it in the water.