The current company I'm working for, VC funded startup, does two company wide offsites per year, 5 days (one work week) each. Then another department wide offsite for another 5 days in another quarter, then the final quarter is a team-wide one that is up to each individual team to schedule and intended to last 4-5 days. So that tracks pretty closely.

TBH I feel like it's just a little much, but not much more than a little much. In other words, I think probably two weeks (10 full days) is probably a good minimum because I do see some value in seeing the team once or twice a year in person. So somewhere between 10-20 full days is probably the sweet spot for me.