usually not very long, if you are really tring to hatch it... ie, Youare not doing trainer battles or wild pokebattles. I suggest using the area from Mauville to (forgot town name to the left of Mauville???) past the daycare... generally aroung 20-30 minutes while traveling on a mach bike fo rmost eggs, some less, some a little more.... Just keep collecting eggs (if you are breeding multiples anyways), and riding back and forth. It doesnt have any grassy areas in your way, and after you beat trainers for the first time, it is a virtually straight shot, and egg hatching is a breeze....