J-31 might only have one prototype, but how much of the development is shared with the J-35, which has multiple prototypes in testing as we speak?
How much won’t be borrowed from the J-35?
A lot of the F-35A and F-35B share most of the same systems. I’d assume it’s a similar concept between the J-31 and J-35. Wings and landing gear (plus tail hook) will be different, but weapons integration, radar, avionics, etc. should be similar. China may have an export approved version for Pakistan, but that may also be similar to what has been exported on the J-10CE, or a better version.
While also doubt it come in 2 years, considering how much has to be done to train the crews, both pilots and ground crews, and well as build out the ground infrastructure to support these jets with stealth coatings, 4-5 years is not an unrealistic expectation.
I also think the PAF is waiting for the PLANAF to field the J-35 actively on their carriers and work out to e bugs while also letting the WS-19 know engines mature enough to open up the full flight envelope of the design. Supercruise, better TWR ratio, a refined weapons bay design to hold weapons beyond the PL-15, or to be able to hold as many PL-15 and PL-10 as to make the jet very capable in A2A combat. All could be worked out in the next 4-5 years, considering how quickly China is advancing.