machinebird
Sorry, don't know how to crop from your post.
You asked 'how long before you get rusty on IFR flights. For me (50 years old. 10,000 hours) I get rusty after a long weekend. No kidding.
The more recent I am (today is good) the better I am.
Time constant? I don't claim to know.
That's an easy question to ask, but a very difficult one to answer.
My point is simple though.
Pilots should not rely on the autos to do stuff they CANNOT do themselves.
By all means use the autos, but only because you CHOOSE to, not because you NEED to.
I'm an experienced Capt. I see F/Os (tomorrows Capts.) who actually fly 1% or less of what they log.
Dammit, I'm not much better than them (but at least I have some experience of hand flying)