I think I would be okay with any term between 2 and 8 years, provided AAV would be 5.5 or less in longer deals. Tippett's 8x6.2M$ might be still a close comparable to Marchenko when it comes to potential long-term extensions. Tippett's scoring rate was slightly higher at 23 than Marchenko's so I don't think Millstein has leverage to ask for more than 6M per year.
I get the concerns about his defensive impact. His effort level in d zone has been inconsistent. He's not the quickest guy on open ice which causes him trouble while defending the cycle. Sillinger and other forwards with short first strides struggle with similar issues. But in close quarters on stick-on-stick battles he looks great, he can strip players off the puck with ease anywhere on the ice, and in that sense I'd say he's a very promising player defensively.
This was his first full season in the league and he played 2nd most minutes of Jackets forwards. It looked like he was out of gas at times, especially in the 2nd half of the year. Maybe he will be better prepared physically this year which could have a positive effect on his defensive stats. We will see.
But even if he was never going to be more than an average defensive player, a long-term deal could still be a worthy gamble, because imo he has the talent to turn into a 70+ point man and be net positive player anyway. He's just about to turn 24, seems like a humble and level-headed guy with leadership qualities, likes it in Columbus, and has no concerning injury history. I don't see a huge problem here.