Here is a table of the activity periods and time it will take you to achieve each rank:

Period  x  14Days  =  Potential Highest Activity
1   X 14 = 14
2   X 14 = 28
3   X 14 = 42---  Junior Member (1.5 month & Minimum Activity 30 & Minimum Merit 0)
4   X 14 = 56
5   X 14 = 70  ---  Member (2.5 month & Minimum Activity 60 & Minimum Merit 10)
6   X 14 = 84
7   X 14 = 98
8   X 14 = 112
9   X 14 = 126--- Full Member (4.5 month & Minimum Activity 120 & Minimum Merit 100)
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--- Senior Member (8.5 month & Minimum Activity 240 & Minimum Merit 250)
19 X 14 = 266
20 X 14 = 280
21 X 14 = 294
22 X 14 = 308
23 X 14 = 322
24 X 14 = 336
25 X 14 = 350
26 X 14 = 364
27 X 14 = 378
28 X 14 = 392
29 X 14 = 406
30 X 14 = 420
31 X 14 = 434
32 X 14 = 448
33 X 14 = 462
34 X 14 = 476
35 X 14 = 490----  Hero Member (1.3 years & Minimum Activity 480 & Minimum Merit 500)
36 X 14 = 504
37 X 14 = 518
38 X 14 = 532
39 X 14 = 536
40 X 14 = 550
41 X 14 = 564
42 X 14 = 578
43 X 14 = 592
44 X 14 = 606
45 X 14 = 620
46 X 14 = 634
47 X 14 = 648
48 X 14 = 662
49 X 14 = 676
50 X 14 = 690
51 X 14 = 704
52 X 14 = 718
53 X 14 = 732
54 X 14 = 746
55 X 14 = 760
56 X 14 = 784 --- Legendary rank (2 years & Minimum Activity between 775- 1030 & Minimum Merit 1000)
For more detailed explanation please visit this link Table of the activity periods and time it will take you to achieve each rank