They shouldn't be in studio if not inspired. Go fishing or walk the dog or something, read a book, get out of town, go to art gallery, see a film, get inspired then enter studio.
But before that... get in rehearsal room and play the songs until they get the hunger to record.
One more question on this: How not to miss the moment when the musicians are 'ready'.
I feel like if it (preproduction/rehearsals) will last too long they might lose some freshness that you are itching to hear from the record..
I am currently getting the band ready for recording their first EP (mixing demos at the moment) and thinking how to fuel them up right before the session so they'll never feel like the songs are not new to them on the session.