How long to wait between coats of polyurethane?
We cannot really answer without being there and evaluating the base wood but under 6 hours is cutting it very short. Have I ever done this? yes but it was 4 year dried walnut that was stained ~ 60 days beforehand close to 2 months. 
My first 5 layers were cut to thin it down to allow it to fill the small divots and grain, yes this may have been excessive but the next coats were full strength with very fine steel wool at first and later 1200 grit wet dry paper. It still looks awesome 20 years later, but if not allow to fully dry it may bubble or craze. The problems may not show up until the next year on a hot day, I have had that happen, had to strip almost down to bare wood and start applying fresh coats to make it look right. I still have those tables and other than a slight yellowing they are fine after 30 years (not in direct sun).