It’s up to you!
The program is designed to be completed in 21 days. This assumes that you complete the tasks in order, and on schedule, completing 1 day’s tasks each day.
However if you take a break, miss a day, etc. then you can pick up where you left of at any time. That said, the program works best when momentum is generated, and kept. So if you do fall behind for any reason, to re-engage, and re-commit as soon as possible to generate the best possible results.