This is not much different that a Stage play. Figure one double spaced page per minute including the "Stage direction", foley, sound effects, and music... --don't for get the music and foley [the sound effects of walking, etc]... To get an Idea of what its like,  listen to The BBC http://www.bbc.co.uk/worldservice/programmes/world_drama.shtml

The Beeb had a competition last year and http://www.bbc.co.uk/worldservice/specials/1521_int_play_comp/page7.shtml gives some of their suggestions.

or listen other "old radio" shows... there are some stations in the US that broadcast old shows from the "golden era" of radio.

Gook Luck!
Joel