Depends much on temperature and humidity. The previous coat has to be hard before it's re-coated or you'll trap solvents in the coating and it will never cure.

Varnish dries with a gloss finish, you want the gloss it reflects sunlight, but you can't re-coat over a gloss finish, has to be sanded between coats. If it gums up the sand paper when you're sanding, it's not dry enough yet to put on another coat. I prefer wet sanding, easier to see quickly if you've sanded the surface or not if you squeegee off the extra water, then if the surface is all dull you're done and if it's not you can quickly see where you need to sand more.

Bill H.