When should I stop SP farming?

This is entirely up to you. If you can take a rough guess at how high you want
each mastery, you can stop SP farming as soon as you get enough to bring each
mastery close to your desired level. Below is a list of how much SP is required
to get to each level in ANY mastery class.

(Information provided by w_tigers87)
Mastery SP Req Total SP Req
1 0 0
2 1 1
3 1 2
4 1 3
5 2 5
6 2 7
7 4 11
8 5 16
9 6 22
10 7 29
11 9 38
12 12 50
13 15 65
14 18 83
15 21 104
16 24 128
17 30 158
18 35 193
19 41 234
20 47 281
21 53 334
22 62 396
23 71 467
24 80 547
25 89 636
26 98 734
27 108 842
28 117 959
29 126 1085
30 135 1220
31 144 1364
32 158 1522
33 172 1694
34 185 1879
35 199 2078
36 212 2290
37 226 2516
38 239 2755
39 253 3008
40 266 3274
41 280 3554
42 299 3853
43 318 4171
44 336 4507
45 355 4862
46 374 5236
47 393 5629
48 412 6041
49 431 6472
50 449 6921
51 468 7389
52 494 7883
53 520 8403
54 546 8949
55 571 9520
56 597 10117
57 623 10740
58 649 11389
59 675 12064
60 700 12764
61 1079 13843
62 1368 15211
63 1420 16631
64 1482 18113
65 1545 19658
http://www.gamefaqs.com/computer/doswin/file/930711/43300