A Samsung Turkey executive tweeted a Nougat release schedule last week which suggested that the Galaxy S6 would receive this much-awaited firmware update in Turkey by the end of this month. Some were skeptical about the timeline but there should be no doubts remaining now that Samsung Turkey has published a detailed Nougat roadmap on its website. The roadmap includes the Galaxy J5 (2015) which means even it's going to eventually get Nougat.
The roadmap mentions that the Galaxy S7 and Galaxy S7 edge would receive the Nougat update in Turkey by the second week of February and we know that to be the truth now. It mentions that the Galaxy S6 Nougat roll out will start in the final week of this month. The roadmap lists devices like the Galaxy Note 5, Galaxy A (2016) series, the Galaxy A (2017) series, Galaxy J7 Prime, Galaxy On7 and more will release days stretching out all the way to November this year.
Naturally, the release timeframe is going to vary by market, but this does give an idea of when Samsung intends to roll out Nougat for the aforementioned devices this year.