How soon before release does a ROM usually get released?

Just as the topic says, i am wondering how soon a ROM is usually out before the release, as i can't wait for SoulSilver any longer :'(

No set time. It depends on the people who actually go out and buy the games.
The process is explained here:
https://www.romulation.org/forum/index.php?topic=395.0

its very rare for a rom to be available before release.