When did Far Cry 5 start developement?
I feel like knowing that answer gives us a better idea about the scope of the game. Assuming producers don't work on 2 games at once, a couple of things can be assumed by looking at Linkedin accounts. 1st, that Primal was made between November 2014 to February 2016, a span of 15 months(1 year, 3 months). I pulled that number out based on Vincent Pontbriand's Linkedin profile. He's only been Senior producer on 2 games, AC Unity and Primal, and specifies he only began work on Primal in 2014 and that must be after Unity.
2nd, Patrik Methe was the Game Director on Far Cry 3 and 4, so clearly he's trusted with the series. Moreover, since December 2014 he's been working on an unannounced title. That's a span of 2 years, 6 months(still ongoing) compared to 4's 2 years, 1 month and 3's 1 year, 11 months. So if he is working on Far Cry, this should be the biggest title in the series. What if it's an unannounced title? Well, according to Ubisoft's financial call they plan on releasing 4 AAA titles by March 2018. We know those 4 games are almost certainly AC:Origins, The Crew 2, Rabbids+Mario and Far Cry 5. So whatever Methe's working on has to be released this year to be considered an on time release