It really depends on a couple of factors.
maybe the biggest factor is the complexity of the cloth, how many polygons are involved in collision calculations; if you are doing a animation with only a simple skirt for instance, you have a relatively low poly object only colliding with the hip & upper leg area of your figure. quick.
A more complex cloth which collides with everything but the head and feet is going to require big calculations for every frame, which is going to take forever.
I guess, as with everything else, it would help if you more clearly defined what exactly you are trying to do.