there is no wearing in period, you put it on and your good too goo, if its squealing try going to your auto store and get some belt dressing, it can be called belt dressing or silicone spray, or you can try WD 40 but go with the belt dressing first
or a pulley needs some lubrication ha idk, open up the fender liner and take a look at how all the pulleys are spinning, if anything take the belt off and spin each one by hand and see if there is any resistance
Yes, really. Belt dressing is a fancy term for belt lubrication. Older V belts didn't mind slipping a little when you "dressed" them. Modern belts are designed for very long service life and are "serpentine" i.e. designed to bend either way. These belts are designed to run without slipping. It is the slipping that causes the squealing and is an indicator that the belt is wearing out faster than it should. Dressing that belt doesn't stop the slipping it just stops the noise caused by the slipping. In other words, belt dressing just allows the belt to slip smoothly instead of in rapid starts and stops which causes the squealing sound (tires squeal for the same reason and you wouldn't dream of lubricating your tires to stop the noise, and brake pads also squeal for the same reason, rapid sticking and slipping, and you wouldn't lube the brake pads to stop that noise either....).
Never lubricate any belts on a modern car, they will slip.