should be fine when it is fully cured.  but that should be well before two to three weeks.

i recently refinished a boat and was varnishing within 48hours of epoxy applied in a relatively hot environment.  clc, in its shop tips http://www.clcboats.com/shoptips/finishing-tips/varnished_kayak.html says it can be ready for varnishing in as little as 72 hours.

you can check that the epoxy is properly cured....no stickiness, tackiness, and if you push your fingernail into the epoxy, you should not be able to make a dent. 

the most important thing before varnishing is the proper prep of the epoxy surface.   the link above takes you through the steps.   follow them and you will have a great looking varnished kayak

Careybob,

Before you run away from the previous post to grab your varnish, keep in mind that the epoxy brand is also very important. Some brands can take many days to achieve full cure, even at high temperatures. Check your epoxy manufacturer's web site to see what they have to say.

Another approach is to skip the varnish for now. Do all the sanding and other prep just as if you were going to varnish, then roll on a very thin coat of epoxy and tip out the bubbles with a foam brush. This will give you a nice 10 foot finish (from 10 feet it will look perfectly varnished). Now, take the boat out for launch and sea trials. People at the dock will be impressed and you'll get to find out how it handles and if all the hardware is in the right place.

Once you've paddled it a few times, bring it back into the shop, make any changes that you need to, sand that outer protective coat of epoxy off and THEN give it the coffee table finish. Think of how annoying it'd be if you realized after varnishing that you needed to move a padeye or something. You'd have to redo a major portion of your finish job.

This way, the epoxy gets a chance to do its full cure, you get to get on the water soonest and only have to varnish once.

Have fun,

Laszlo