I don't know about hours but it is longer than what you might suspect. I believe the primary reason is that each bag is x-rayed for contraband before it is put on the belt. If they see anything questionable they mark the bag with chalk and put it on the belt. Then, as you walk out past customs, they look for any bag with a chalk mark.

A colleague of mine in New Delhi tells me he looks at his bag when it's on the belt and if it has a chalk mark he brings it into the men's room and wipes it off before going past customs.