Since some of the combat j7 and j8 are fairly new built, just 5 or so old, (plus there are some recce and training variants that are even newer) it is likely we will see both j7 and j8 in service for another 20 or so years. Albeit in ever-dwindling numbers.
j6 aren't in service, as far as i know. there are roughly 200 combat j8 in service plus a few dozen recce variants. There are also some 700+ combat j7 in service plus over a hundred training two seater jj7. It should also be mentioned q5, also an old design, is still in service. some 150-200 combat planes plus several dozen two seater training variants (which are pretty new and could also serve some 20 more years)
Roughly, one could expect some 60-80 of these old planes (total, not of each kind) to be retired each year.