None. You need to read the fare rules for the flight.

Your best bet is to search "Unrestricted" fares using the advanced search on the United website. That normally means no change fee, no cancellation fee, and refundable to the original form of payment. However, you should read the fare rules before purchasing.

Stay safe.

ETA: But check the "Flexible" column too. Those aren't necessarily fully refundable, but they might be.