Informally I consider 15-20 minutes to be OK and any later to be embarrassing. However, it depends on local norms and rules, just like the standards for how long a restaurant reservation will be held when people are late and don't call.

In Japanese hotels, there can be a computer system that supposedly forces front desk clerks to charge a fee for checking out even a few minutes late, although calling the front desk before the checkout time if you'll be slightly late supposedly helps. This has been my experience in a couple local chains in the better business hotels to three or four star range.