A History of Banwaves

A banwave happens when Blizzard comes up with a way to detect bot software. Often they've done this through updates to their Warden program which polices the games for bots. 

Major banwaves have happened on:

Nov. 16, 2006 (followed by a smaller wave in December)

June 11, 2007

May 20, 2008

It seems to take Blizzard a long time to create the software that will allow it to detect Glider or other bot programs and once they've banned thousands of people, the bot programmers quickly go to work creating updates that prevent detection. 

There's a great summary of the 2007 and 2008 banwaves on Serial Ganker's Blog.

I lost my first main account in the Nov. 16, 2006 banwave and created a new main account to play in Burning Crusade. I botted a toon up to 70 and stopped botting that toon in March of 2007. When the June 11, 2007 banwave hit, friends of mine lost their accounts but my new main account and another account on which I'd botted up to May were both untouched. 

This suggests that there is a 1-2 month window outside of which your previously botted account can be considered clean if you stop botting.