I'll note that PC's can complete a subsector in about 2 years...
Maybe. Then again, I'm a pessimist.
let's take the most obnoxious: Lanth...
Lanth includes Stellatio, one of the Marches' "deep" systems. It's over 46 days at one gee from the stellar 100D limit to the mainworld and I don't know if that exceeds the 0.2 c hull thickness imposed velocity limit.
A survey takes a group a week, give or take a few days...
Size, atmosphere, hydrographics, and population can be done in orbit. Worst case is nine hours for population, but you can scan all simultaneously if you have the bodies.
After landing, you can rate the starport with a 3 hour walk around.
Worst case for the government audit is 54 hours but you aren't going to be doing that around the clock. You're going to be interviewing officials and whatnot by comm and personally during their working hours. The Assistant Vice Deputy Under-Poobah isn't going to be answering your questions at his version of 2am so you're looking at ~5 working days.
Law level is even worse. Worst case there is 108 hours. Again the interviews will take place during working hours so we're looking at ~10 working days. Of course, two players or groups could audit the government and law level at the same time.
All that presumes you get the interviews and contacts you need immediately too. Will those IISS papers open every door you need as soon as you knock?
Tech level is a real puzzle. (What else is new?) Three or more hours per item, but how many items need to be examined? Again, a player or players can be auditing TL while the government and/or law level interviews are taking place.
Because of the interview requirements, worst case I think we're looking at ~2 standard weeks on each world.
Given that there are probably a half dozen teams per subsector, and the data is cross-checked...
That's how the project will really get done. While the 20 year time limit is an issue, the need for cross checks demands it.