Automix: Option to indicate the average playtime of a track

It would be nice to have an option where I can indicate the average time I want a tracks to play. Now, with longer tracks (e.g. 6 minutes or longer) the automix mixes out at the end, meaning that the tracks plays too 'long'. Usually,I want to play a maximum of 1,5 to 2 minutes to keep the energy of the mix up.