A sequence in a radio play might be one line long, or last for 20 pages but no single sequence should go beyond its natural length. Beware of boring the listener.
Radio is fatally easy to turn off.
The only means of establishing a character's presence is to have them speak or be referred to by name. If there are too many characters in a scene the listener will lose track. If it is important it should be there in the dialogue. Sound effects should be used sparingly. They should work with the dialogue.