Android N won't be out till later this year for Nexus devices. After that the carriers / Samsung will have to get it and modify it how they want so it will be some time.
Typically for Samsung, you'd wait at least a month after official release of N to Nexii. That's if you're not using carrier locked devices. If you are, you could wait for half a year while the S7's from the rest of the world laugh with their newly upgraded S7.
AT&T has now surpassed Verizon as the slowest carrier for updates, so I wouldn't count on it. However, they are aGSM network so most unlocked phones will work on it. It's possible to source an international unlocked S7 and get it to with on AT&T ater jumping through some hoops.
Posted via my Droid Turbo 2 that I wish were unlocked
I would say between now and then. LOL or between now and 2025. Its ATT so i had to add extra time. sorry coulndt resist. Someone asking about an update OS that hasnt even been released yet.