Is 3 days 40 days Chillah compulsory:No anything is accepted

......................It is not anyway compulsory and rather it is routine 
that because of our other engagements many of the jamaat does not spend full 3 days.  
At some places even it is permanent (like some campuses where there is 5 day week) that Jamaat goes for 2 days only, at some institutions specially in Islamic Madarsas it goes from Thursday Asar to Friday Magrib only. 
At Aligarh Muslim University, the system is as follows : Jamaat goes on friday after juma they return on Saturday morning for attending classes and again go after classes till Sunday evening/ Monday morning.
(As for 40 days) again it is not mandatory I have gone in jamaat for 10 days, 13 days, there is nothing fixed, Sometimes brothers go in jamaat after giving competitive exam as they are waiting for results they come back after declaration of results for taking admission or attending classes, may be at 6,8,24,31…………………………. or whatsoever number of days. Work has an open door for all 24 hours 365 days. Anyone can check these facts.
As Sheikh Salih al Uthaiymeen has also said
With regard to limiting da’wah to three or four days, or to two or four or six months, or two years, there is no shar’i evidence for that. But they think that this has to do with organization. If a person goes out for three days, and knows that he is limited to these three days, he will focus his mind and forget about worldly matters. This is the matter of organization, and is not an act of worship or a matter of sharee’ah. .....................................
....................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. 
As all of us has family and Job commitments so if you are going preplanned it becomes comfortable as you can apply for your leave for certain number of days ,Your family will remain informed that you are returning on a particular date. Likewise so many system benefits are ther. 

On the other hand I know brothers who give more than three days a month may be 6, 8 or 10 days in a month. Similarly more than 40 days in a year may be 2 months 3 months 4 months in a year. Allah has said in quran that I have bought the Jan and Mall in exchange of Jannat)
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.