Well, using the Captivate as a case study ...
Froyo released May 2010
Captivate launched with Eclair in July 2010
Captivate received Froyo in February 2011
So while Froyo had already been out for 2 months when the Captivate launched, it came with Eclair and didn't receive Froyo until 7 months later. This time, the GSII would have been out on AT&T for about a month before ICS hits a device.
So I'd put an official ICS push to this device sometime in June 2012 (if ever).
If you want it sooner, there will very likely be a developer release that you'll be able to flash if you're rooted.