According to a recent Mint article, you should be looking at your purchases at least every two weeks! Why?
Banks tolerate credit card fraud – they say it's too expensive to be 100% sure. Meanwhile, we all pay for fraud – and it can happen on our credit card.
You only have 60 days to dispute a purchase. If you are not checking your purchases until you get the bill at the end of the month (or, like most people, even less often than that!), you might easily miss the 60 days.
If you don't know it's coming, you can't budget for it – that's what can happen with fraudulent charges if you are not checking. That is, you will just be charged.