If I had to put it into a number: about a 7 or 8
Pros
-Stupidly productive through 3 seasons.
-Most durable RB in the league. Has led the league in carries per game 3 consecutive years, and the only thing that's cost him games is the commish. Feature-backed for two 270+ carry seasons at Ohio State and remained healthy there, so he is now at 5 straight years with no injury.
-Elite endurance. Runs even better in the 4th quarter than in the 1st. Rushes on fresh legs in December. Can handle an extremely high workload and remain productive.
-Has produced behind top notch OLs and more average ones. Not dependent on the line to perform. Could produce in any offense.
-Has underutilized receiving chops. Caught 77 balls last season, which proves that he at least he at least has good hands. Has only run one route deeper than 10 yards in his entire NFL career, and that play wasn't even called by his OC. Still has unexplored receiving potential.
-Excellent pass blocker. Combined with his receiving skills, he is totally functional in the passing game.
-Running style is built around strength and vision, and is not based on elite athleticism. He could lose a step to age or injury and still remain productive.
Cons
-Has not been injured yet, but plays a high-injury position. The risk is lower with Zeke than most other backs, but it's always there.
-"Wild child" who's had problems off the field since his Ohio State days, and hasn't showed signs of learning yet. He's not playing through a 4 year contract without getting in trouble, I guarantee you that.
-His attitude towards conditioning has been lackluster, notice how he always plays a step slow in September. He can play his way into shape in his early 20s, but it's going to become more of a problem as he ages. The cliff can come fast for players who don't stay in shape: see Darrelle Revis.
-Plays heavy to soak up hits, which costs him speed. We'd have to dial back his workload for him to play at a more natural weight, but then that becomes the question of are you getting your money's worth from him.
To sum it up, I'd say I am much more worried about Zeke off the field than I am on it. On it, he's a top 3 RB who can be fed until he's sick and efficiently converts that into monster stats. Off it, he's reckless and kind of lazy. I have minimal concerns about his game on the field, but I think his habits off the field will eventually catch up with him.
So for the short term - say 2019 and 2020 - I have great confidence that we could get every dime worth out of him. Longer term is when things become more cloudy.