Coins are able to be used for minting after only 30 days, but the actual time it takes to be rewarded for a proof-of-stake block varies based on the total coin-age and the number of other people currently minting.  Regardless of how long it takes, however, you will eventually receive a reward equivalent to ~1% per annum.

There are several detailed discussions of how Peercoin minting works over at the Peercoin forum including this Minting Guide: http://www.peercointalk.org/index.php?topic=2982.msg29805#msg29805

Also, an online calculator has been developed to help estimate the probability of minting a block with a certain set of coins over a certain time period: http://peercoincalculator.info

Good luck finding your first POS block!