From a coaches point of view, the man can easily dominate till he is 35.
From a fans point of view, I would say quit once he turns 31-32. The reason he packs stadiums is the fact that he is going to win in an impressive fashion. Once he starts to age and become just your average world-class sprinter, the aura that is Bolt, will be lost. That will give him nearly 10 years on top, assuming no phenom is coming up the ranks. He will have made his money and demolished WR’s so that aspiring youths will have superhuman times as goals.