that because of our other engagements many of the jamaat does not spend full 3 days.
....................and you go out with them in order to purify yourself, there is nothing wrong with that. There are many people whom Allaah has guided at their hands.
Al-Baab al-Maftooh, no. 10, P. 304.
Say for example .. a group of say 5 brothers were sent to Calcutta from Delhi and nothing is fixed, and one brother said he has come for 3 days, two for 6 days, one for 20 days and another 40 days.
It would be rather problematic. Actually if you sent them anywhere it would be problematic.
This is the reason that a fixed time is specified so that it is easier to organise a jamaat so that they will be together all the time,
However there is no restriction on going for 1 day / 2 days / 4 days / 1 month etc make the responsible brothers aware the time that you intend to go for so it's easier to send them in an appropriate jamaat.