its the most aggrevating this ever. Its an 05 S the guy at the dealorship said it should take 1 hour???

 

probably less than an hour but you'll need something similar to this tool to release the tension on the belt...
Assenmacher Specialty Tools , MC103 Mini Cooper Serpentine Belt Tool
I haven't needed to do this job yet, but I'll probably weld together something that will work before I spend $50 on the tool!
A long (2 foot) flat bladed screwdriver will probably help a great deal as there is very little space to work the new belt around the pulleys. Also, be sure to sketch the routing of the old belt before removal to make install easier...good luck!