Lowest activity to become a legendary is 775 activity and there's at least 1 user who became legendary with only 775 activity.
Here's a post by theymos back in 2015.
Here are the luckiest and unluckiest Legendaries so far:
+-----------------------------------------+--------+
| realName | alimit |
+-----------------------------------------+--------+
| ajareselde | 780 |
+-----------------------------------------+--------+
Thanks for the info list, guess i forgot that i was pretty lucky as well to reach legendary at just 780 activity points, but then again; it's not as nearly as
interesting like it was when the membership was introduced because there's a ton of legendaries now :/
Congrats on the rank, welcome to the club.