https://www.federalregister.gov/docu...turbine-engine

Due to Covid issues and the wearing of masks above FL250, the FAA amended the regulation (14 CFR 121.333) to stipulate that one crew member does NOT need to wear a mask, if the mask available is a quick donning type, up to FL410 for aircraft having a seating capacity of 31 or more (or a payload of more than 7,500 lbs), or FL350 for aircraft with 30 passenger seats or less, or less than 7,500 lbs payload.

If one crew member leaves the cockpit, the other does not need to wear the mask, below FL410.

This applies to Part 121, found in 121.333:

https://www.ecfr.gov/cgi-bin/text-id..._1333&rgn=div8