Sigma90
Brain of Cthulhu
Well... if I'm going to hypothesise...Eeew. Then it could be almost '19 by the time that happens. No offense, just trying to be realistic.
1.3.6 was originally slated to be a bug fix, but threads like this prove it's evolved beyond that. Having said that, I do think it will be primarily bug fixes, with a few new furniture sets and maybe some other small content. I don't expect anything large such as new biomes or new events.
Re-Logic were advertising for beta testers last week. From that, I would assume 1.3.6 is almost in a state it can be beta tested. Allowing time for recruiting new testers, letting them do their thing and fixing any issues that arise, I would expect 1.3.6 in early December. A couple of hotfixes to 1.3.6.3 and then the team will probably take Christmas / New Year holidays off.
Console 1.3 release will happen around early November, based on the update "working through the final issues/bug list".
The next version released for PC/Console will be 1.4 and will be the first of the One Terraria releases. I expect the long-rumoured Creative Mode to arrive then. I'm guessing a release mid-year 2018 for that -- most of the effort being Creative Mode. I don't think it will be too time-consuming to port over the PC content to Console since most of it is 'same same but more' -- more furniture, more armour, more blocks... there's nothing significant like new biomes or new mechanics.