I also check once a day (usually at night when I'm transferring all my financial transactions for the day into Quicken)
Tools - Extensions - Get More Extensions - Scroll through to find the Adsense notifier - Install - Close FF and re-open it. Enter log-in details. Hey Presto!
It depends. I normally check it every few hours a day to around twice a day but if I am working on my site where I will relocate and touch up the ads, then I will look at it every half an hour or so just to see if my revenue is on the increase or if I have done something terribly wrong with the ads that I stop earning money.
Well, IMO it's clever to check often, let's say once or twice a day (but better more than less), since that way you discover fraud-clicks before it's too late ... tayiper
If you don't have firefox you can use syssense. It sits in the system tray and you can set it to update whenever you want. You can download it at my site (shameless plug)... http://www.thephotodoctors.com/syssense.htm
Yea, good idea to use something automated to stay consistent and to quickly discover problems (or hopefully dramatic increases). AdSense Detective comes with something for this too if you don't have something custom.