Steam can refuse a refund after that time, but they are usually incredibly flexible because a) they want to keep customers on Steam and b) many jurisdictions have much firmer and consumer favoured laws around product refunds, Australia for example is a large reason for Steams current refund policy in the first place.

2 hours max for a guaranteed refund, anything else (within 2 weeks) needs to be approved by a human to make sure you're not just beating the game and returning it after.

Steam are usually nicer with games that is not officially released yet.