how often does time machine back up under big sur?

Time Machine automatically makes hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for all previous months.

Check more in Apple's article: Back up your Mac with Time Machine - Apple Support

Like the other have stated it has not changed except TM does not appear to work normally with mail. With mail open cannot scroll back in time to a specific email as before. This is a Catalina carryover issue. You can still go to your mail V6+ folder and find backups but just not as efficient..

Time Machine has always backed up hourly.