This is the 3 basic example to know Rank updates.

The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)

For example, if in a 14 day period you post 14 posts, you gain +14 activity.
If you post 2 posts in a 14 day period, you gain +2 activity.
If you don't post at all in a 14 day period, you don't gain activity.
However, if you post 15 or more posts in a 14 day period, you still gain only +14 activity.

This is the basic explanation for jump ranks.
1 X 14 = 14
2 X 14 = 28
3 X 14 = 42     ( jr member) 30days-42days
4 X 14 = 56
5 X 14 = 70     (member) 60days -70days
6 X 14 = 84
7 X 14 = 98
8 X 14 = 112
9 X 14 = 126   (full member) 120days-126days
10 X 14 = 140
11 X 14 = 154
12 X 14 = 168
13 X 14 = 182
14 X 14 = 196
15 X 14 = 210
16 X 14 = 224
17 X 14 = 238
18 X 14 = 252 (Sr. Member) 252days
19 x 14 = 266
20 x 14 = 280
21 x 14 = 294
22 x 14 = 308
23 x 14 = 322
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35 x 14 = 490(hero  member) 490days or 16 months