Well, the least activity required for legendary is 774. Which boils down to 56 activity periods of 14 points each (totaling 784 activity). If you register on the last day of an activity period you "safe" 13 days. If you bank a few posts you also get all 14 activity on the first day of the last period so you "safe" another 13 days. In total you need 758 days of consistant posting or 2 years and 28 days.

The highest activity required for legendary is 1030. Which is 74 activity periods of 14 points each (totalling 1036). Again you can "safe" as above so a total of 1010 days or 2 years and 280 days.