Rockbox

Since most of us are neither psychics nor mindreaders we cannot decipher clearly what it is that you want.
First: what is a "mix"? Is it an ordinary playlist? a playlist with shuffle? a playlist with weighted black magic shuffle?
Second: what should the player do when the limit is reached? power off? (There is already a sleep timer option available htat does this)

My apologies... "Random Mix" is a term from the Karma, and I probably should have been more descriptive.

It would just be an ordinary playlist, containing random tracks, calculated to last for a set amount of time.

For example, say I have a collection of 500 albums that contain a total of 5,000 tracks and all happend to be 5 minutes long. I would then select "Random Mix" (or whatever you want to call it), and then I would enter the length of time I wanted it to last, say 1 hour. Rockbox would then randomly select 12 songs and create a playlist. After the playlist is over (in one hour in this case), it would do whatever it does now when a playlist ends... presumably just wait for user input.

I hope this makes things clearer...

Yes, that makes it much clearer. Currently rockbox does not have any way to tell the
length of an entire playlist, but I know some people are interested in that sort of thing.
I think this is possible to do, lets see if someone wants it bad enough to implement it :-)