How many times should he call a non-Muslim to Islam?
Question

                For up to many times should we conduct Islamic lectures to a group of non-muslims who seems not interested to what we are conveying. We are a volunteer dawah brothers and we use to visit workshops, factories, hospitals to make dawah. Oftentimes we make a visit which last 4 to 5 times in one place. But we noticed nobody is listening. Some only stayed just because the owner of the establishment ordered (obliges) them to attend.
Praise be to Allah.We ask Allaah to reward you with good, and to bless your work and your efforts. 
There is no specific number of times that you should give lectures to those whom you mention. That depends on the number of places you visit and the time that is given to you, and the extent to which the people are responsive. The daa’iyah is required to be wise in giving da’wah. The Prophet (peace and blessings of Allaah be upon him) would try to choose the right time for preaching to his companions so that they would not get bored, and this is how you should be. Strive to call people to Islam in a way that is better, and choose suitable times and places for da’wah. 
The daa’iyah should not despair if he makes da’wah time after time and sees little response. Hearts are in the hand of Allaah, and all the daa’iyah has to do is to convey the message. A word that the daa’iyah thinks little of may have an effect on a person’s heart years later, and may be the cause of his being guided. 
In order for this word to bear fruit, it should be accompanied with a smile, compassion and a sincere desire to save these poor people and bring them forth from the darkness of kufr and atheism.  If the daa’iyah can add some money or help to his words, let him do so, for that give credibility to the values and principles to which he is calling people, and may prepare people’s hearts to accept the goodness that comes to them and remove any hatred, resistance and objections. 
Also strive to solve any problems that these people may have with their bosses at work. This will have a great effect in softening their hearts towards the daa’iyah. Long ago the poet said: 
Be kind to people, you will win their hearts… 
Even better than that is the words of Allaah (interpretation of the meaning): 
“The good deed and the evil deed cannot be equal. Repel (the evil) with one which is better (i.e. Allaah orders the faithful believers to be patient at the time of anger, and to excuse those who treat them badly) then verily he, between whom and you there was enmity, (will become) as though he was a close friend”
[Fussilat 41:34]
You should leave with these people something that they can look at in their spare time, when you are not there, such as pamphlets and booklets. If a person thinks about the truth when he is on his own, he is more likely to accept it and submit to it. 
Part of your da’wah should be addressed to the bosses themselves, advising them to treat these workers well and to show them the good morals and attitudes of Islam. Often that is a cause of the workers embracing Islam, just as the bad attitude of a boss and his mistreatment of his workers may be a cause of their resisting Islam. 
May Allaah help you to guide others and make you truly guided. 
And Allaah knows best.