I've been collecting from about 60-70 percent of my friends on my "initial friends run." I don't really have any new rules, but I try to visit at more "peak" times when I think people will have been on recently. There's no formula, but I probably won't be visiting any of my UK friends later in the US evening, since they are probably sleeping (except neo NEVER sleeps!). I've been making 2 or 3 attempts at visiting my friends from my main account each day. I don't think I've cleared everyone; there will always be a few regulars that have been busy for the few days.
Here's the link for my "heart payout rate" data collection thread; it's still a very small sample size.
http://forum.ea.com/eaforum/posts/list/0/9276801.page#27026639
