I'm a testing fanatic. I really like having those colorful little tubes to convince me that what I'm seeing is right. But like Byron said, as time went by I became really familiar with the fish that I keep, and the way my tank behaves - they're all different!
It's tricky in the beginning, but now that I've become familiar enough with the specific fish I keep to know what normal looks like for them (and unfortunately what they look like when something is off), I don't worry so much, and don't test as often as I did when I was starting.
That said, I do still test fairly frequently with new setups until they become more established, and whenever I add stock to a tank or QT. Generally, if I'm testing one tank, I go ahead and do them all at once - because why not? I'll also test after I do something that isn't absolutley routine, like if I've changed a filter pad, or done a particularly heavy cleaning. . .
So like Jaysee said - whatever you're comfortable with - and that will change as time passes :)