Short answer: As long as it takes.
Longer answer: It depends.
Much longer answer: It depends on the individual, their learning capacity, number of times a week they practice, their attitude, etc. etc. etc.
Some take anywhere between 8-10 years, others take 20-30, some take a whole lifetime.
Not just in aikido, any martial art.
BTW, don't you mean "exponent", not "combatent" [sic]?