Could someone please tell me the approximate time to generate an address that begins with 5-12 custom address.
You've already got a few answers (multiply by constant for each additional character), but if you plan on offering this as a service to others, you might want to take care of a few caveats. Note that the best way would be to check the vanitygen source and look at the actual code used, duplicating that in your service.
Caveat 1: Make sure you disallow the characters 'O' (uppercase 'o'), '0' (number zero), 'I' (uppercase i'), and 'l' (lowercase L), as they're not allowed in addresses. Make sure they know what characters their address should start with ('1' for Bitcoin), etc.
Caveat 2: Some prefixes are harder than others; https://bitcointalk.org/index.php?topic=301068.msg7478542#msg7478542
Caveat 2.1: Prefixes that are run-on digits of 1 are especially hard:
1AAAAA = 259,627,881
111111 = 1,099,511,627,776
Caveat 2.2: Some first characters are harder than others:
1 = 256
2-6 = 23
7-P = 22
Q = 65
R-z = 1,353
Caveat 2.3: Prefixes alphabetically (digits first, then uppercase, then lowercase) before the pivot prefix '1QLbz7JHiBTspS9' (it's longer, but this number of characters will do for practical purposes well into the future) are easier than those exactly on it, and easier still than those after it:
1QLbz5 = 259,627,881
1QLbz6 = 259,627,881
1QLbz7 = 837,596,142
1QLbz8 = 15,318,045,009
1QLbz9 = 15,318,045,009