biggirl...I've been keeping track on how often a PR comes out. I stopped doing this about two months ago after almost 14 months keeping track. What I've found is that the median number of days (not an average but frequency occurences) for a PR is every 6 days (I'm counting the weekends). Most (about 85%) PRs come out within a 16 say time span. The longest was like 35 days. Tomorrow will be day 7 since the last PR on May 2. Most likely we'll see a PR this week. But since we're in a quiet period, until that ends we'll see some real meaty PRs.
Go back far enough and you may see one of my PR statistics near the beginning of each month.