Elementary probability theory.
Each position has 1/58 probability to be the desired symbol.
So if you fix let say 3 positions the probability to find a matching pattern would be (1/58)*(1/58)*(1/58)
It is independent of the position you fix in the address. 1ace....xxxx is as difficult as 1axcxex.......
The possibility vanishes very quick as a power of n(positions you fix)
Thanks @qdoop!
That's a quick and concise answer!
FYI, the tool is called vanitygen.
findftp@mint16 ~/Downloads/vanitygen-master $ ./vanitygen 1abcdefg
Difficulty: 51529903411245
[195.63 Kkey/s][total 16791552][Prob 0.0%][50% in 5.8y]
findftp@mint16 ~/Downloads/vanitygen-master $ ./vanitygen 1abcdef
Difficulty: 888446610538
[221.12 Kkey/s][total 1468160][Prob 0.0%][50% in 32.2d]
So just a "simple" prefix takes a long time on a intel xeon w3505
edit: must say I run this linux on software vmware.