Some rehearsals are short – all that may be needed is for the MD and the singer to get on the same page regarding keys, song structure, arrangements.
Other rehearsals may be longer – for example a vocal ensemble may need to sing a piece over and over again together, to learn to come off each note at the right time and together, to listen over and over again to each other so that each person knows exactly where their part fits in the arrangement, to get tight.
Some rehearsals are all about feel – a new band may need to play and practice together for a sustained period, to get to know each other, to get used to the nuances of each other’s playing, to begin to learn to read each other’s habits.
Other rehearsals may be more technical – following charts/scores, making sure everyone is executing what they should be.execute the arrangements together, as a group.
If you know what type of rehearsal yours should be and what you want to achieve in it, then demand that, just that, and save everyone some time.