driving to Milan

Go to viamichelin, or even mapquest to get a good estimate.

5 1/4 hours is a good estimate.

You have two options: you can drive to Cesena on the SS3bis route and the follow on a long straight route to Milano, or go via Betolle and Firenze. They take equivalent time under normal circumstances. Off-peak I'd go via Firenze, peak time would make me take chances with SS3bis (which has more trucks, but avoids Firenze altogether).

Take the E45 freeway toward Cesena. There is hardly any traffic there, I take it all the time from my country house. Shortly after the Cesena exit you will connect to the Autostrada A-14 in the direction of Bologna. At Bologna stay on the lanes taking you onto the Autostrada A1 to Milan. Unless you run into some accidents along the way you should be at the Milano San Donato exit in just over 4 hours. Then from there to the city center will depend on traffic, maybe 20-25 min outside of rush hour.

I wouldn't go through Florence, first of all because it's longer under normal circumstances, but also because between Firenze Sud and Calenzano there is a lot of local traffic on top of long distance travelers, since Florentines use that stretch of A1 to commute. Also the Firenze to Casalecchio stretch is curvy and with hundreds of big trucks. If there is an accident involving some big rigs (very common) you are toast, you'll be stuck on the Apennines for hours.

You will also save on tolls. The E45 from Perugia to Cesena (a 4 lane divided highway) is free all the way to Cesena (then from Cesena to Milan you have to pay toll). Watch out for the speed limit on the E45, which is 90km/h (although I always drive well above it and I never got a ticket there so far).