Perhaps the user will find some insights from a link below
Back up your Mac with Time Machine - Apple Support (CA)
Excerpt from the link provided
" Time Machine makes hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for all previous months. The oldest backups are deleted when your backup disk is full. "
Is is usually good practice to have the Designated TM Backup drive with at least 2 1/2 Greater Drive Capacity than the Internal Drive.
This should give enough space on the TM Drive for a good amount of time before TM Backup begins to Delete Old Snap Shots.
Addition reading on TM Backup - external link
https://www.macworld.com/article/231067/how-to-delete-time-machine-snapshots-on-your-mac.html