It's a good idea, but is there a way to make it come out to exactly the time you've specified? I get approximations but I was hoping that iTunes would poke around and try various combinations until it hit exactly the (in my case) 30-minute mark, but no dice. Am I missing something?

No, but it's a good idea.

I just come up with a way to get as close as possible to the time limit, and it's really obvious so I don't know why it didn't occur to me earlier:

I wanted a playlist to last an hour, and using

Limit to - 1 hour - selected by - Random

I couldn't actually make it hit an hour--it was always about 57 or 58 minutes--so finally I tried

Limit to - 61 minutes - selected by - Random

and that didn't work, so I typed

Limit to - 62 minutes - selected by - Random

and voila! 1 hour on the nose. I even added up the times manually (yes, really) and it was 60 minutes and 5 seconds. Near enough for me. Maybe nobody else will care if it's a few minutes short, but if I say I want an hour, I want an hour, and since it always undercuts, the secret is obviously to overstate by a minute or two.