if your epoxy is cured to the point that it is not plasticy (e.g., you can sand it and/or press it with a nail and not dent it) you can go ahead and varnish.

this can be in as little as 24 hours with the right temperature and asuming you have not made an error in your epoxy mix.

that said epoxy wil continue to cure for a while, but you will not notice it and it won't be effected  by varnish.

my general practice, is i let a hull bake a week before my final sanding prior to varnishing.  its not totally scientific but is based on the final shrinkage that occurs in epoxy as it approaches its final cure - from ~99.9% to ~99.99% which happens after it is cured enough to take sanding.  but it is not a requirement to wait.

h