Is there a 100% completion estimate for MOUSE: P.I. For Hire?

Not from the developer, but Mortismal Gaming reports about 25ish hours for a 100% run.

Is the official length estimate based on player averages?

No public methodology has been published. The FAQ range is the official estimate, while the 25ish-hour figure is one reviewer’s completed 100% run.

The official FAQ says 12 to 20 hours of gameplay, depending on how you play, and one post-launch 100% reference currently lands around 25ish hours.

The official FAQ puts MOUSE: P.I. For Hire at 12 to 20 hours of gameplay, depending on how you play. That is still the baseline for a normal run.

For a fuller clear, the clearest post-launch reference so far is Mortismal Gaming, which says a 100% run took about 25ish hours.

MOUSE: P.I. For Hire currently looks like a 12 to 20 hour game for a normal playthrough, with one concrete 100% reference landing around 25ish hours.

The developer FAQ still gives the cleanest official baseline: 12 to 20 hours of gameplay, depending on how you play.
Mortismal Gaming finished the game at 100% and reported spending about 25ish hours on that run.
Taken together, those two references point to a normal story run on one side and a noticeably longer completionist route on the other.

There is still no official completionist estimate from the developer.
We still do not have a broad player-average split for story-only, exploratory, and 100% playstyles.
There is no official chapter-by-chapter breakdown.

If you push the story and ignore most cleanup, the official 12 to 20 hour range is still the safest expectation.
If you slow down for side jobs, collectibles, and optional cleanup, expect the total to move past the FAQ floor and toward the mid-20s.
Because mission replay is limited, completionist runs can take longer if you are protecting manual saves and backtracking through earlier slots.