How long should oil-based polyurethane dry before I pour epoxy over it?

Give the final coat at least 24 hours; 1 to 2 days is typical. You don't have to wait for the poly to cure all the way through. Once the last coat has dried for a day, UltraClear Bar & Table Top Epoxy can go on.

If the polyurethane is there to seal an oil-based stain, apply a minimum of three coats of oil-based poly over the stain before you count the drying time. Oil-based stains take a long time to fully dry, and epoxy poured directly over stain that isn't completely dry can fail to bond and peel off months later; the three coats of poly seal the stain so that doesn't happen. Alcohol- and water-based stains are the faster route: they dry quickly and the epoxy can go over them in about 24 hours with no poly needed.

After the poly has dried, lightly sand it with 220–320 grit to remove the gloss, clean off the dust with acetone, let the surface dry, then brush on the epoxy seal coat and pour the flood coat as normal.

The poly doesn't have to be fresh. A bar top that was wiped with oil-based poly a week ago, or a piece you finished earlier, is ready once it's clean and dry. Only oil-based polyurethane works this way; epoxy won't adhere to water-based poly.