Cap data sheets do not spec "burn in" time to reach rated capacitance. PP caps do not dry up like electrolytics so there is no dielectric reform to worry about. Once you put the caps back in they will perform the same way they did all these years right off the bat.
Stored, used, crossover capacitor burn in / reforming time
2 years ago I pulled out a pair of 24 uF metalized polypropylene capacitors from my speaker crossovers. They had been in use for 15 years. I just put them back in for comparison with my replacement caps . How many hours would they take to fully burn in / reform again ?