Me bessell and porter went years ago.
Id definantly recommend a week. Barce is a BIG place..so much riding to be had and 3 days just wouldnt be enough.
Also, there are various trials parks..all dotted about, but all a train ride away..so for example if you were staying in barce town, you would have to get a train heading out , to go the parks ( about a half hour hour journey ) and if your staying near any of the parks, youd have to do the same trip back.
Barce is awesome.. Go when its cheapest.. Weather is pretty decent all year round.
When in barce ( nights out and stuff ) just be wary..Lots of pick pockets and muggers and horrible people.
Other than that. Enjoy.