I like to keep my queue updated constantly, but as a commissioner AND an artist who takes commissions I think it really comes down to keeping everyone informed at all times, but also individually if any issues come up. I like to show preview sketches to make sure everything is good, as well as other stages of completion if asked.
I feel that a lot of tension between artist and commissioners can be solves through communication, but also smart payment plans; I take payment AFTER the sketch is approved, because it means that I don't have to worry about getting scammed after already completing a picture if they turn out they can't pay, AND the commissioner knows I already have progress on the picture so they don't have to worry about waiting a year before seeing any progress (hopefully).
:D

As a commissioner, I feel happy as long as an artist communicates in an honest fashion. If they make a deadline, they stick to it, and if there is confusion about the nature of the work or specific details, they ask before charging blindly ahead.

WIPs and preview sketches are also nice to see, just so I don't get the finished work and have to point out some glaring error that should have been addressed in a much earlier stage. It makes me look like a douche.

Tip: don't give out deadlines if you can't keep to them

"It'll be done by tomorrow!" that looks good if it really is done by tomorrow. But it starts looking quite bad when it's almost 2 days later by now ;)

I'd suggest being more nebulous, unless you know you can 100% certainly be done by the deadline. Eg, "it'll be done soon". That gives wiggle room to when you actually finish, but still lets the commissioner know you're working on it and haven't forgotten them. :)

Or even, "sorry, I'm working on it, but don't know when it'll be done" if you really don't know at all when you get the chance to finish it. It lets the commissioner know you're thinking about them, and you're being honest about the situation. (Now, I know you're not intentionally trying to deceive anyone, but failed promises look like that from outside even when you make honest effort. So if you're bad at estimating, it's simply better not to try.)

I, at least, don't mind waiting for a bit for my works, so by not making any definite promises that are too soon, you're not failing to keep them and everyone's happier. :)

(In my personal experience and expectations, anything sooner than 2 weeks is fast delivery. 2-4 weeks is pretty common and ordinary. Longer than that is slow, but all of this also depends on the complexity of the work commissioned of course.)

Sorry, noting you now.