It depends:
If it's your pokemon without items then 3 days or at least 8 hours.
If he has Lucky Egg as held item, then 4 -5 hours or 2 days.
If your pokemon has lucky egg and was traded, then 4 hours maximun.
Of course, all this in the E4. Sinnoh's E4 is the best to level up pokemons(2nd round)