RedCardinal - 10:03 am on Sep 16, 2012 (gmt 0)
In a webmaster Central Hangout within the past couple of weeks JohnMu said that all manual spam penalties were automatically released after time. I think it was 1 year max. He explained why also.
If you search the YouTube channel you will likely find the Hnagout on Air. He uploads these under his own username.