How Soon is Going Away Soon?

Potentially never
it will wipe suggestions and deleted items first.
The only time you'll start to see deletions is when you start using KUID (keep until I delete) aggressively.
That being said 72% is a bit high for me, I try to run at 50%

In your OnePass Manager, set the 1P to "Keep at Most:" to "All". Then change your User Preferences, Recording Options to "Keep at Most:" to "All Shows". Do that and you will never see a yellow dot again.

I have never used KUID. But I do check the last (bottom) of my Recently Deleted Folder. When the last program gets killed off, your disk is full. Then I delete a month's worth of programs.

If you want to see a curious part of those programs, check their "Keep Until" date. It moves every day.

More trivia: If you have two TiVo boxes and transfer a program to a different TiVo, it will be set to KUID after a month.

For shows without a hard limit (keep at most 5, etc..) - that date is the units best estimation of when the show will be deleted for space. Actual deletion time could vary any time from "soonish", to "never". It's, not the worlds clearest mechanic, assumes you never manually delete anything, and doesn't scale well with larger hard disks.

The simple answer is, when you run out of space, it will go deleting things to get more space (only as much as it needs). The dates in 'going away soon' are just guesses.