I generally do not check on them at all. Let them be until finished. Lenght of that depends on style, wheat beers 2 weeks, most others 3-4 weeks. I have my process down well enough that I do not bother with unrequired gravity readings. Just do one at the end when I bottle/keg. Honestly that's not really required at all either. Once you have been brewing a while, you realize that the yeast know that to do without much help from you. They are also very good at what they do.
"Look, let me explain something to you. I'm not Mr. Lebowski. You're Mr. Lebowski. I'm the Dude. So that's what you call me. That or His Dudeness... Duder... or El Duderino, if, you know, you're not into the whole brevity thing." -The Dude