Chilla 3 day 40 days 4month Tablighi Jamaat Reality and Myth

1. This 3 day and 40 day is just a system and a (Tarteeb) for management purpose like any other system.And tabligh jamat dont go outside just for dawah. Dawah is only part of it. Mostly it invest time with improving ownself, getting knowledge, just like madrassah. Those days amounts are fixed just for management sake. 
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. 

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.  

1. 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. 

2. At Aligarh Muslim University,Aligarh India  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 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.