The more the ply, the more expensive it gets. It all boils down to your road grip and driving style. Usually the tyre tread is a good indicator of tyre use. Someone who has driven 20-25k in city and someone on off road in a village will have a different experience of tyre condition.
I have kept Honda Deluxe longest upto 36,000km and then sold off the bike still on reasonable stock tyres. The tread wasn't that bad at 35000km, but the bike had lost its grip unlike in its earlier days or maybe that’s what I felt. However mechanics still thought I could easily cross 40k. Frankly you won't find a black n white answer to your question either. So if you feel your bike has lost the grip after 20-30k driving, then replace the tyres if that satisfies you and not sooner unless the signs of wear tear are too prominent.
