It all depends on the priority of the coaching staff and front office.
If I were in charge, I would give him all of this year and all of next year. I'd be relying upon a quick passing game along with the run. I'd be trying to figure out what he does well, what he doesn't do well and try to improve all of that over the course of the season. Wins and losses would not matter to me. If I lose by 100, I lose by 100. My only concern would be did the QB get better from the 1st quarter of the season to the 2nd quarter of the season and so on? I don't believe at all that that is this coaching staff's focus and that was my biggest worry when they went defensive minded head coach.
I believe they're trying to maximize wins at the expense of developing the QB. It's absurd that 92%+ of the play calls inside the redzone are runs. It's not playing to develop the QB, it's playing extremely conservatively and trying to avoid mistakes to win games. It seems obvious to me that they're wanting to maximize wins over all else which is why I'm surprised they just aren't playing Siemian with that being the goal. Entering today's game, the Bears were running 66% of the time in a pass first league. Today, 22 passes (including the 5 sacks), 40 rushing attempts. It's football numbers wise from a different era.
Fields looked terrible in his limited opportunities today, no doubt. He looked terrible last week too. I think the writing is on the wall that they'll be picking a new QB next offseason. I base this on current usage as well as based on how little help they got him this past offseason. Fields may well suck but he's been set up to fail by 2 staffs now who seem more concerned with maximizing wins than they do about developing the QB.