I think a realistic timeframe is about 15 years from the time someone releases a viable tool to complete replacement of the last engineer. About half way through the pressure on salaries will become noticeable. I base this guess on how companies typically operate and how long practical adoption of smaller technology changes take.
In the first half there should be both a downward pressure due to the threat of replacement but at the same time an upward pressure for the same reason. New grads will stop coming and existing engineers can look forward to being phased out and command a premium for as long as they can.
Another possibility is that it doesn‘t quite get good enough, but regardless students start picking other subjects, creating a temporary shortage that sustains existing employees. Employees will just be reduced through retirement with half being retired after 20 years of this process anyway.
Yet another possibility is that a half-assed tool only reduces demand - again that can be fixed through retirement plus less or no new people joining the field.