Banned as in blacklisted by Google? As far as I know if you're blacklisted you remain blacklisted - it's permanent. I think.
Just make a Google Webmaster Central account and analyse why you got banned. Then fix the problem and send a reinclusion request. Then you can get back in the search engine. It can take a while, there is no standard amount of days for this to fix it as far is i know. Good luck!
If you are banned by duplicate content, hidden text, doorway pages, etc, you can fix these and reindexed by google again. For some others like cloaking, illegal, and harm google profit, etc, it will be permanent.