It kind of depends. You can obsess over your SERPs on a daily basis and put yourself in a paralysis of analysis, when you should be spending the time building backlinks or writing content. I use Advanced Web Ranking to check all of my keywords 3-4 times a month, a little more often if I am trying out new link strategies, less often if I’m busy with more pressing matters, and then may check an individual keyword or few on a daily basis just to keep up with where things are at. Analytics is another beast that can cause it’s own paralysis if you look at it too often, still I try to check in on my traffic on a daily basis along with the sources and keywords driving the traffic as well as conversions.
Bottom line, check as often as you feel comfortable, but make sure you aren’t stuck looking at stats for hours when you could be doing something more valuable for your website.