As a baseline answer, the Royal Australian Air Force has a rundown on the recruiting page, hidden under 'full details' option and 'training' where after completing initial flight training they have:
Pilot Advanced Courses (1 year). If you are streamed to go down the FJP pathway you will be required to complete the following courses:
Pilot Advanced Course (6 months). Remaining at RAAF Base Pearce you will undergo training on the Hawk Lead-in fighter at Number 79 Squadron (79SQN).
Lead-in-Fighter Course (6 months). Once you have learnt the basics of flying the Hawk and operating at higher airspeeds, you will be posted to 76SQN at RAAF Base Williamtown to learn how to operate the Hawk in a tactical environment (air to air and air to ground).
This is training on the Hawk, a fast jet with weapons capability but not a current combat aircraft. Then they have
Operational Conversions (6 months). At the end of the advanced courses you will be streamed for operational conversion onto one of the following platforms:
And then listing the F18F, F18G and F35
So the starting point for a hypothetical pipeline for pilots to effective fighter pilot is 18 months. While a war footing might let you strip weekends and other down time out it seems unlikely you could do better than 6 months. In practice I would suspect that the key factor in this timelines is training assets - do you have enough simulators, instructors, training aircraft and a safe places to fly for all your pilots, for Ukraine I'm sorta expecting the answer to most of above is 'no'.
So if you need highly effective general purpose pilots you need to have started a year or so ago. If you are at the point of sacrificing one or more of 'effective', 'general purpose' and 'pilot' then other things become possible.