It all depends on how he handles the physical play over time. So far, so good, he is pretty feisty for a little guy and if he can keep it up and stay healthy he should be fine. The difficulty is that his offensive acumen and accompanying performance in pressure situations (albeit a small sample size so far), his price tag may skyrocket pretty quickly. And with a bigger price tag comes bigger minutes. There's a difficulty in a third pairing PP specialist pulling down 3+ mil or whatever might be what's coming for him. If he turns out to be a Rafalski he'll be here for a long time, you just don't let those go. If he's MA Bergeron he's gone pretty quickly.
He's in a contract year now, right? If he doesn't have an absolutely stellar year or gets horribly exposed up until an extension is worked out, I'd figure he'll get a bridge deal of 2-3 years at about 2 mil. He's on 1.7 including bonuses now if not mistaken. It's what happens after that deal that is hard to predict as of now. Seids and Chara might well be gone or on their last legs, Hamilton possibly a stud or maybe even a dud. We'll know more about Krug's durability and consistency.
If this was a competition I'd say 7-8 years. He seems to fit well here, is a warrior and plays a way bigger game than his stature. If he can keep that up hell be here at least until we can't afford to keep him which may occur when he's approaching 30 and looking for a big payday.