i have an LO similar age who is also a catnapper. once last week he took the first long nap of his life and i was actually worried about waking him! ha!! no worries, he woke at 2hrs exactly!! clock changed and ping! eyes open.
i would say if you have a chronic catnapper, let him go 2.5 hours, but i'm no expert, that's just my gut. but, i would also take into account how it will affect timings for his next feed, next A time, a later catnap and bedtimes, etc and go from there.
sorry my lo is ill and i am rocking him and typing one finger so can't type more!