A few notes first. There is a similar blog a while below, and it uses Algebra to determine when it will catch up. I speculate based on the events that have happened in the comic and are have yet to happen in the show. I am making a similar blog about this, because for some reaosn it won't let me comment there.
So, based on how much story they could fit in a season (adding more side stories of course, like they did with the flu at the prison), I believe the show will catch up with the comic at the end of season 10. Here is why:
Season 4, part 2
They will be on the road. We will deal with some new characters, the group finding each other, and by the end of the season they will have found Father Stokes and will have killed the hunters. (I believe the hunters will eat Judith)
Season 5, part 1
They will continue on the road, Morgan will be introduced, they will deal with the DC scavengers and they will meet Aaron and Eric in the mid-season finale.
Season 5, part 2
They will go to Alexandria, they will get to meet the people there, find out about Davidson, find jobs to do, have some personal drama etc. Rick will deal with the pete situation and in the midseason finale the safezone will be attacked by the scavengers like the comic. Rick will become the leader after that, but I think it would be better to have elections rather than have Douglas make Rick the new leader.
Season 6, part 1
The herd will arrive and the safezone will be cut out from the outside world. Eventually the walls are going to come down, the herd will swarm the place and many will die. People will work together and deal with them. Rick will talk to Carl (who is gravely wounded) and tell him about the world he wants to build, and that the zombies are a manageable threat, due to the extended rotting (more years have passed in the series than the comic). The midseason ends by showing jesus watching over Alexandria.
Season 6, part 2
Jesus appears, Rick takes him hostage, but is eventually convinced to go to hilltop. He agrees to help against the saviors, and fights some on his way back. Someone gets killed Abraham style by Dwight, and rick returns. Some more events unfold, and the season ends with the introduction of Negan. Someone gets Lucilled like the comic. I'd like daryl because Glenn is not as important as his comic counterpart.
Season 7, part 1
Rick has submitted to Negan. Negan comes for the offerings, while Rick is working with the hilltop and the kingdom against Negan. The midseason ends with Negan saying "We are going to war".
Season 7, part 2
Rick appears in front of negan's base, they negotiate and negan's base gets swarmed like the comic. The rest of the outposts are also taken care of throughout the rest of the season,and the season ends after negan gets out of his base and attacks Alexandria. The "unthinkable" happens in the season finale.
Season 8, part 1
The story picks up right after the unthinkable. The war continues for the first half of the season.
Season 8, part 2
By the time the second half comes out, it will be february of 2017 (most probably). at least more 50 issues will have come out until then. I'd say it will take two seasons to cover these.
Season 10, part 2
By this time, another 25 issues are going to come out. This is where the series will catch up in my opinion.
So, what do you guys think?