In the worst case, you would get 2 leap seconds per year. So in the worst case, 1800 years.

However, in the past 50 years, we've only had 27. So at the current rate, probably more like 6667 years to be off by an hour.

Of course, if the earth's rotation changed speed by so much that 2 leap seconds per year wasn't enough, we'd find a way to cram more in. In fact, in that case, the case for leap seconds would probably get stronger.