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)

The above is simple table to see when your rank will change, for more detailed explanation please visit this link Table of the activity periods and time it will take you to achieve each rank