Is there a way to change the Windows 7 desktop background rotation time?

I'm assuming it's a registry key but I haven't been able to find it yet. You can use Itty Bitty Wallpaper Rotator to achieve the same effect though. It allows you to enter any custom time you want and works on XP and Vista as well. Simply tell it which folder contains the wallpapers, set any additional options, and hit Rotate Now. Easy as that!

Itty Bitty Wallpaper Rotator is free and open source.

HKCU\Control Panel\Personalization\Desktop Slideshow\Interval, the value is in milliseconds, but from my testing, setting this other than a value in the dropdown will reset it when the drop down is opened.

Update: It works as long as you don't reopen the personalization window, then the interval will reset and you'll have to set it in the reg again.