Well, It's not uncommon for filters to last more than 10,000 kms. It depends on the conditions in which you live. If the air is mostly clean and dust free then I don't think even 15,000 kms should be a problem. I personally change it at 10,000 kms but that's mostly because they are not that much of an expense. If you don't feel any difference in the car's performance then it should be OK.
You won't actually damage your engine if you continue to run it. The worst that will happen is that your airflow will become restricted and fuel economy will suffer a little.