David--

The longer you can wait the better. Some degree of curing may go on for a week or two especially if you're working in colder temperatures. I guess an easy answer is that if you can sand it and not have it pill up or clog your sandpaper, its probably OK to varnish. Given a choice, I'd wait a week.

Some people like to paddle their kayaks for a month before varnishing to give the epoxy time to fully cure. I shy away from that, having sanded all the UV-clouded epoxy off the deck of a kayak once. The damage didn't all happen in a month, but you can see clouding sooner that you might expect.

Mike Scarborough
Friendship Kayak