| I think it takes longer on the way down, assuming friction is present. When going up there are two components of forces opposing its motion - both gravity and friction are causing it to slow,so ut will slow pretty quickly. But on the way down gravity is causing it to accelerate while friction is causing it to go slower. Hence the sum of the forces when going up is a greater opposing force than the sum of the forces on the way down. This means that the deceleration on the way up is greater than the acceleration on the way down.
You can see this perhaps clearer if you think through a couple of examples. The example that OneGuy described is correct if there is no friction present. But imagine the case where the friction force is equal to or greater than the gravity force - in this case after the object moves up the ramp and stops, it never stars sliding back down, so here the time it takes to slide down is infinite. Thus as long as riction is present, it takes longer to go back down the ramp than it took to go up and come to a stop. | 
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