I dont believe a 23 year old plane would normally be retired. I would think that, in the given procurement scenario, majority of the 1200 planes would remain in service.
It really does depend on whether j10 is some sort of semi-interim solution before 5th gen one-engine fighter gets produced or are there no serious plans to replace j10s any time soon. If china wants to catch up - former route is a must.
I would think that production rates for previous airplanes like j7s and j8s - when they reached they peak, could serve as a decent guideline. j11s have been under production for quite some time now but they've never really gotten over 2 a month limit. Sure, its a bigger plane but also in a way more old fashioned.
If i must give a figure, i'd say production will max out at around 50 a year, and that might happen in the coming years, certainly before 2010. Which would then mean some 700 or so planes produced by 2020 by which time i'm hoping 5th gen plane will start production instead of j10s being produced. Actual replacement of then existing j10s by new planes i dont see before 2030.