If your objective is to have degrading resolution of snaps as time passes, might I suggest instead - run 3 'sets' of snaps. A '15m' schedule, a 'daily' scheduled, and a 'weekly' schedule.

And then automate the deletions in each with e.g. 'find' - when you're >95% work through each, and purge the '15m' backups more aggressively than the daily/weeklies.