Production on Apple’s next-gen A15 chips will reportedly start up in May, using the same 5-nanometer fabrication size as last year’s A14 chip. However, this year’s A-series chip refresh will boast an “enhanced” manufacturing process that should result in superior performance next to the A14 processor.
This is approximately what you would expect to see in terms of both the schedule and nanometer jump between iPhone models. Apple typically is underway with production by around May, ready for the September iPhone launch. There is also usually an upgrade (or, in terms of minimum feature size, downgrade) in nanometers every couple of years.