I brewed a peach wheat beer last year, I had it in the primary for a week and then racked on top of the peaches in the secondary and left it for 2 weeks. I had them in in a muslin bag to keep the beer cleaner. I used about 8 lbs of peeled and pitted peaches in a 5 gallon batch, it had a good mild peach flavor, not too overpowering.