There's no one answer. As a founder, you want to set aside the smallest possible amount for the option pool, because this option pool dilutes YOU, not the investor... and if you don't use it all, the "leftover" options benefit you AND the investor, so the investors get some "negative dilution" that they never earned.
This is all explained clearly by Nivi in The Option Pool Shuffle.
I successfully argued for a 10% option pool in my series A on the grounds that we didn't need to hire too many people. The best thing to do is make a list of new employees you plan to hire in the first 18-24 months (not founders) and how many shares you need for each.
If you DO need more options later to make a key hire, you can still issue them, it just dilutes everyone, which is fair.
The typical employee stock option pool is 20%. It might vary a bit but it's wise, at each round, to set aside 20%.
For empirical data on option pool size and equity allocation, check out compstudy: https://www.compstudy.com/ The 2008 summary report is here: http://www.scribd.com/doc/7494620/2008-CompStudy-Report-in-Technology 15-20% is typical.