I did an AHS anniversary stout, added 6 oz (by weight) of dutch processed cocoa powder during the boil and then after 2 weeks our so, I added 4lbs of frozen red cherries (from costco) that I heated up with little to no water until it hit about 170F and held it at that temp for 5-10 mins. I keep it on cherries for about 2 weeks. I have more detailed notes as to when I racked onto fruit and how long it was there for at home (stuck at the DMV :/ ).
Ironically, after it had been on cherries for a bit, it had gotten infected but I'm almost positive it was NOT from the fruit addition but from a fly that ended up in my bucket after fermenting in the basement (there were also more than a dozen tiny dead flies in the starsan in my airlock which leads me to believe I may have gotten a suck back).
Either way, it's got quite a thick pellicle on it now after 5 months of being infected but tastes great and smells even better. It's not a strong cherry flavor but it's still there. I think a little additional sugar In the form of lactose would bring it out more and balance real nice with the roastiness.