What would be Duchene's case for getting that sort of an AAV ? He's averaged 55 pts per 82 games the previous 4 seasons - seasons that would be expected to be his peak point producing years given that he's turning 28 this coming season - and spent chunks of those seasons playing on the wing.
Ryan O'Reilly is the exact same age as him, has scored 62 pts per 82 games over the same period, is 100% a centre, and one of the strongest two-way players in the game and he makes 7.5. And even with that, many people think of that as being not a great value. Other than being a former 3rd overall pick and having the lingering prestige that, for whatever reason, seems to stick to players as a result, what compelling argument could Duchene mount for being paid more than that?
Ryan O'Reilly signed years ago. His cap hit percentage is 10.5, meaning the equivalent contract to O'Reilly's under a 79.5M cap would have an 8.3475M AAV. That's pretty much in line with what the other poster is suggesting when they say 8-9. I'd argue that the poster was a bit high with 9M, and the range is more 8M - 8.5M, I'll explain why below....
That's not even considering that O'Reilly's contract is structured in a way that almost all his salary is paid in signing bonuses. This likely gave the Sabres a slight discount on O'Reilly's AAV since players typically settle for a bit less for that sort of consolation. We might give Duchene similar signing bonuses to Bobby Ryan, but judging by the fact that we've never given out a full signing bonus contract, I doubt we're giving him that sort of incentive.
Johansen is another comparable, and he got 10.67 percent of the salary cap.
O'Reilly and Johansen's contracts buy up more prime years than Duchene's will. Couture just signed an 8 x 8 extension that won't start until he is 30 years old. Couture's cap hit percentage is 10.06 percent which leads to an 8M cap hit. Duchene will be about 2 years younger than Couture. Couture also got an extremely strong NTC (can only name 3 teams to get traded to) and a total of 16M in signing bonuses over the first four years of his contract (including lockout bonuses).
Because of the age difference with the first two comparable contracts and then the Couture contract, I think market value might be somewhere in the middle of the two different contract types. This tells me we're looking at a market value of between 8M and 8.5M x 8 years with some sort of signing bonuses. Couture got 16M of signing bonuses, which would be in line with what Duchene would get if we gave him similar bonuses to what Ryan received, which was 2M per season.
Granted, Duchene has to want to stay here, and he is also in a similar situation to Bobby Ryan where we gave up so much for him that letting him go in a trade or as a UFA would be disastrous. So he has the leverage to get above market value.