I would let the beer sit on the peaches for at least 1 mos if not more. 1-2 wks is relatively short for such a high gravity brew to blend with the subtle flavor of peaches. I recommend adding some extract flavoring for additional flavor/aroma at packaging time (but not the whole bottle).
Fresh peaches are definitely the way to go if you can find them cheap. Don't worry about the pasteurization method as that is a big PITA. Simply clean them and depit them well. Cut them into chunks with a sanitized knife, and place them in a freezer bag ( I spray mine with starsan). Freeze for at least 2 days to rupture cell walls and reduce microbes on the fruit.
When you are ready to use them, boil some water in a small kettle (that will fit all of your frozen peaches) to sanitize the kettle and lid. Dump the water, and add your peaches and put the lid back on. Let the peaches defrost (this will take a few hrs and can be sped up by letting them defrost slightly in the bag first). With a sanitized funnel (I use a small liter plastic soda bottle, cut in half, fits right in the mouth of a carboy) and scoop add your peaches into the carboy. I then purge my carboy with CO2 to reduce oxidation in my beer. Carefully rack the beer on top of the fruit (don't be afraid to suck up some yeast to help ferment the fruit) and then purge the headspace again. This technique minimizes oxidation occuring to the beer before secondary fermentation of the fruit takes off (which can take a couple days sometimes). Let the fruit sit. The longer the better.
