That depends on whether you're type 1 or type 2 and lots of other variables. First of all, you should always check it in the morning before eating. Then, two hours after eating is a good rule of thumb. If you're type 1 and using insulin, before bed is not a bad idea. I'm type two and watch what I eat very carefully because I don't want to get out of control and end up taking insulin. I take my sugar measurement in the morning before eating and the only other time is whenever I suspect something might be wrong. As long as I'm eating right and exercising daily, the numbers vary very little. I'm not recommending this, just saying that good management can allow you some leeway. If you're not sure, stick with what I said in the first part. Beginner's must do a lot of research on their own. Doctors often seem reluctant to spend the time necessary to properly educate you. Can't imagine why... (gotta get that next $patient$ in here).