To earn rank you have to earn Activity point by posting relevant message on any forum thread, but in each period of 14 days you can only earn maximum 14 Activity points so you need about 42 days (1.5months) after posting 30 reply or topics messages here to earn Junior Member rank, please check table below for other ranks, above Junior Member ranks you also need Merit point to rank up as given on the second table below

Here is a table of the activity periods and time it will take you to achieve each rank:
Period  x  14Days  =  Potential Highest Activity Earn
1   X 14 = 14
2   X 14 = 28
3   X 14 = 42  ---  Junior. Member (1.5 month)
4   X 14 = 56
5   X 14 = 70  ---  Member (2.5 month)
6   X 14 = 84
7   X 14 = 98
8   X 14 = 112
9   X 14 = 126  --- Full Member (4.5 month)
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)
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)
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)

Rank			Required Activity					Required Merit
Brand New			0					0
Newbie			1					0
Jr. Member			30					0
Member			60					10
Full Member			120					100
Sr. Member			240					250
Hero Member			480					500
Legendary			Random between 775-1030					1000