Mac storage basics · Time Machine basics
How long does Time Machine keep my backups?
Short answer: Hourly backups for 24 hours, daily for a month, then weekly for all earlier months until the disk is full, when the oldest are removed.
Time Machine keeps hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for all previous months — until the backup disk fills up. When it's full, it deletes the oldest backups to make room.
So how far back you can go depends mostly on the size of your backup disk compared with how much you back up and change. A bigger disk means a longer history; see how big a Time Machine drive should be.