The trip takes more or less three hours, depending on traffic, driving skill and number of stops. Four or more is more realistic, though you could actually do it in two we suppose. Whether your vehicle has a manual or automatic transmission, take descents by using low gears matched to your speed for control. Brakes are for stopping and emergencies. Continuous brake use heats them.
Hot brakes are ineffective.
As well, you may damage them to the point that the rental firm will charge you for the abuse. If you're still uneasy about driving this yourself, or have no car, the Post Bus organization has often run tours including all or some of these passes. Suggest you contact the nearest Post or Tourism Office for information. It's in German, but you can figure out most of it. If you find more that you don't understand use the AltaVista translation facility .