I do not think anyone really knows - most get booted from the index. I have heard 30 days mentioned...

But look at it this way:
1) It is in Google best interest to have as many sites as possible in the index - the more sites, the better quality results (assuming they can rank them properly)
2) It is in Google best interest to have an index and search results that match a searchers query - I carefully did not say "Its in Google best interest to get rid of spam" ... for eg a site that uses a spam method, may actually have valuable content that the searcher is looking for (so having the spam result there may be valuable for the searcher)
3. Based on this, maybe the best way to deal with spam is NOT to boot sites from the index, but set the algorithm to ignore the spam method so the site still has a chance to rank (as they may have value).
4. This way Google ignores hidden text (rather than a penalty), it ignores doorway pages (rather than bans sites for them), it does not index 2 duplicates sites - it just indexes one (which is what it does now), multiple subdomains are ignored and the main domain is only listed (rather than ban them all) etc etc

I have no idea if the are or would ever do this (so do not quote me), but it is possible that spam sites that rank well, actually rank well for legitimate reasons as well.

CBP