If the past is anything to go by, the first news we will hear about PAF J-31s is when they actually land at a Pakistani airbase to be greeted by senior leadership.
When that happens, 2026 or 2030 is anyone's guess really, none of us can be sure. We do know that China can work incredibly fast, that PAF projects with regards to fighter inductions are always kept under wraps, no years of "negotiations", "Cabinet clearances", "approvals" etc like who know who.
As someone pointed out, why would China, possibly at a big cost to itself, equip PAF with such a strategic game changing system? Well the benefits are many.
1) A stealth fighter will be a MAJOR headache for IAF. Especially if AMCA is 20 years away from service and F-35 is no go for India. PAF May be able to strike Indian targets with almost impunity.
2) Pakistan takes it's fighter purchases incredibly seriously. J-31 in PAF service sends a signal that China can and will export Stealth. This message will not be lost of the host of muslim countries denied F-35.
3) A PAF support infrastructure across various bases for J-10s and J-31s suits China just fine.
4) Effectively puts an end to any enduring relationship between the Pakistan Military and the US. By 2035, bar a few F-16s that are new, it is very likely no major US weapon system will be in Pakistani service. Again, this suits China just fine.