How long would you recommend discussing the Bible with someone on social media?

It depends.

Do they seem receptive to scriptural truth?  Are they argumentative and combative?  Do they agree on some scriptural truths but disagree — yet in a respectful, friendly way — in other things the Bible brings out?

If they seem receptive to biblical truths, keep studying with them and pray that they continue to have that open, honest heart (Lk. 8:15; Matt. 5:6).  Steer the discussion, if you haven’t done so already, to the gospel message of God’s love and grace, Jesus’ sacrifice, and how to receive salvation and mercy from God through faith, repentance, and baptism into Christ’s body which is his church (John 3:16; Eph. 2:8; Mk. 16:15-16; Acts 2:38; 1 Pet. 3:21; 1 Cor. 12:13; cf. Eph. 1:22-23; 4:4-6; Gal. 3:27).  That, above all else, is what they need to hear the most if they are not a Christian as the New Testament defines it.

If they are open to some biblical precepts and yet disagree with others…albeit in a friendly, respectful way…then don’t burn that bridge.  Keep studying with them about other matters, and do so with Christ-like love, patience, and friendliness yourself.  Remember, your job is to sow the seed (Lk. 8) and do so in the proper way (Eph. 4:15; 2 Tim. 2:24-26).  It might just be that while they reject that biblical truth now, later on they may repent and accept it…and obey it (cf. Matt. 21:28-32).  Pray for that outcome regularly.

If they are argumentative and combative, and continue to be on a regular basis, then end the discussion (cf. Matt. 10:14-15).  Keep in mind that we all have a bad day sometimes.  However, if you observe that the person is always like that, decide to not even engage them in discussion because you would be wasting your time.  Here’s why (Prov. 28:26; 12:15; 1:7; 18:2; 17:10; 26:1-12).  Keep in mind God’s instruction about such things (1 Tim. 1:5-7; 4:1-2; 4:7; 6:3-5; 6:20-21; 2 Tim. 2:14-18; 2:23; 3:1-9; 4:3-4; Tit. 1:10-16; 3:2; 3:9-11).  If need be, block them so that you will no longer see their contentiousness and get discouraged or upset by it, or be tempted to waste your time in fruitless engagement with them.  That’s really helped me at times.  Life’s too short to waste time trying to change the heart and mind of someone who is stubborn and contentious.

Above all, make sure you yourself are acting and speaking in the right way.  For more information on how to do that online, check out a blog post I wrote about the subject recently.