My current general rules for masking are

1. If I am outdoors in a sparse environment, I do not require myself to wear a mask, but should still try to keep a >6' distance between myself and others. If I am already wearing a mask, whether and when I take it off is up to comfort.
2. If I am indoors but alone in the room, a mask is recommended, as aerosols may still be lingering in the air; but if I figure enough time has passed, and masking is uncomfortable, I'll allow myself to take it off.
3. If I am outdoors in a crowd, or indoors with any number of people (with exemptions for a few specific people), I must wear a mask.

But when it comes to the "sparse" environment vs "crowd" rule... How many grains in a heap, right? I've tried looking into this myself, but the information I find is vague and contradictory, which leads me to feel conflicted in situations where it might be a heap but it's hard to determine.