My logbook shows that many years back it took me:
- 4 weeks to do the first stage on a Flying Scholarship + paid for hours of my own
- 20 days for a holiday in which I did no revision / flying
- 6 days to complete in minumum (then 40) hours
All the exams done around the flying, and while I did not feel particualrly pressured at any time, weather was an issue.
I would say that if weather, aircraft and instructor availabililty, and ability are no issue, I would budget for 30 days. If you study hard / perhaps pass some exams before the 21 days, then certainly 21 days is do-able.
Groundschool was minimal; confuser use was high!