If by Christmas or the end of the season, progress is slow or non-existent, then he'll have been in the job 2.5 to 3 years.
At that point, I think you have to look around to see if there's a better option available at a reasonable price. If there's not, and I thought he was doing OK in an average kind of way, I'd keep him on, but to really improve, sometimes you have to take the risk of getting a decision wrong, so my gut reaction would be to take a risk and try someone else.
We've accepted mediocrity for too long. If I didn't fulfil my part of the bargain on a contract on a regular basis, then said contract would be terminated. Football managers are no different, and I don't believe that the full contract value to the endof 2028/19 would have to be paid out if it was terminated. Normally some compromise is reached.
For the record, I think he'll do better next season and have us in or around Europa league spots. But if he doesn't, and we have another season like this one, then I'd want the board to look very hard for a replacement. Essentially, a big part of the board's job is making the most of the finances we have, and the manager is pretty much the biggest part of that.