Always Something Else..
As an ensemble you play together. Plain logic. That's why you are rehearsing as an ensemble. Preferably regularly. I will not give a firm period for the sessions together but playing and rehearsing together regularly is absolutely necessary. We rehearsed once a week in my guitar duos, with the flute-guitar duo the frequency was much, much less. Never mind, our level of ambition was much lower as well.
Naturally, there must be an opportunity for it, you will have to allocate time. Time which is a rare resource in this busy life! Even if you agree on specific rehearsal dates there may be numerous occasions which foil your agreement. Just think of the contemporary -work ethic' of flexibility and unsocial hours.
As I said before, regularity is quite important for an ambitious ensemble, because after you have prepared your rehearsals at home it is just annoying to find out that a session together must be cancelled.
It is even more annoying if you don't know exactly when the next rehearsal will take place anyway. If each time it becomes more or less of a gamble if the session will be held in the near future. At a certain moment this annoyance will kill your motivation: what's the use of preparation at home if there is no opportunity to see the results of this effort? Decrease of motivation decreases progress and may cause the ensemble's wearing out.
So choose a firm date for your ensemble rehearsals. Select days on which you are the least disturbed by social obligations. Of course, it cannot be prevented that you have to cancel at times. If cancelling becomes the only regularity, however, it is wise to find another date.
If there is no other date? Well, in that case you will have to adapt your ambitions by mutual agreement. If you don't, annoyances will emerge automatically.