I just replaced a broken belt in our Maytag dryer, it was the right belt, identical to the old one, but it's not working so good... It's properly installed, but it's like the belt is slipping when we put a full load in it, we never had this issue with the old belt. Do new belts need some time to break in?
Tighten the spring on the idler pulley.
Or the old belt broke because the bearings are shot (not good, in my experience that's new
Actually on a dryer, it's worth it/much easier to replace the bearings and glides. If the drum doesn't spin freely without the belt, you may want to go ahead and a take a look at the drum glides and see if they're worn or missing, before you burn up this belt too.
Addressing your title question. No, no break-in required - it was better "instantly."
If you only replaced the belt, you probably did not solve the problem; the broken belt may have been a symptom of other issues with the drum drive train, making it harder to turn and stressing the belt as a result.
I just did a similar repair (my belt was not broken - but the drum was not spinning) and I found that a complete kit of belt, new idler puley and spring and new drum rollers was very little more cost than just a belt.
When I pulled things apart, the idler pulley bearing was clearly sticky, and the drum-roller bearings were also quite sticky. For under $30 I had the dryer up and running better than it ever had (we got it used.) It used to squeal a bit on start up and rumble quite a lot all the time - both are gone. I pitched the hopeless parts, and tied up the old belt, writing a note to self (or any future repairer) on the inside of the dryer about the date and model of the replacement kit, and there there was a used, old, but intact belt down there in case a belt does break.