From what I've seen, it can happen like rugerfreak's around 100, or like a few more around 300-400 rounds, but it can still show minor but noticeable peening around 800 or so rounds even. The 800ish round count is much rarer but I do recall seeing it sometime before on this forum, and I want to say a couple people started seeing it +1000 rounds too. But the average I'd have to say is between 300 and 400. Just examine the barrel and slide after each range trip and see what you can determine, if you think you have a peening problem, post pictures please, they greatly help us out in seeing whats going on.....in this case a picture IS worth a thousand words.
But peening itself isn't as often a problem as the forums and internet makes it out to be. I'm not saying it's not a real problem, just one that's been exaggerated by tons of extra posts about it. Most of which are honest questions about it, but if you did a search, you'd see quite a few threads about it, probably more referencing it in someway as a potential problem for a future purchase rather an actual post with pictures about one thats peening now.
Right now it's the biggest problem for the SR9, but it's still a slim chance you'll get one that will peen. If you get one that does, just know that it may take Ruger more than one or two trips to AZ to fix it, but they have been taking care of the ones that do peen.
Josh