If this is to be implemented, I would put something like 6 seasons. Basically if you trained your player fully, you can change him to your nationality and get financial benefits.
If a player is bought as an 18-20y old, he can play for his new NT, as soon as he's ready anyway 24-26 yrs.
If he's bought as a half trained 22y, he can still contribute a lot from his 28th to 32 years
If he's bough as a finished product, in need of some polishing as a 24, he can still play for a couple of seasons.
Players trained somewhere else, bought at 25-26+ cannot be converted.
Less than 5-6 seasons, I don't like it. And I'm not sure if I like it at all...