It really depends on the person's style, what you're asking for, and their workload. Usually, I expect like a week or two from most things. Though if I'm getting a simple sketch, or something else "simple" it could be less.

My personal experience is often 1 to 4 weeks. As long as the artist gives me a time frame up front, I have no difficulty with the longer timeframes.

It really does vary per artist. I like to wait no longer than a month for something I get, but I can wait longer if an artist regularly contacts me or at least posts update journals on current workload status. Transparency is almost as important as the artwork itself in my opinion.

I'm pretty patient so I usually just want a monthly update on it at least so I see it makes progress. I try to be not too demanding because I think people also might have personal duties and such that are very important to them and I'm already very rateful that they use their precious spare time for me. As a supporter I always wish to make a good impression. If someone tells me though that they will have the art finished in a specific time I surely ask for the update on it earlier.
If there is anything I can rather ask for would be the most important then it is the option for asking about possible changes if things are missing in a WIP yet. I think if things are possible to negotiate and if there is a polite conversation along with the planning of an artwork then it is an overall better experience. (^.=.^)

The times i've commissioned you, I've felt you've been rather quick to show results. Granted, on average, the usual wait time for an artist is anywhere from a week to a month or so, some artists work faster than others for various reasons, most people i've dealt with, usually are very patient, but it's still good to communicate with your clients, so they have an idea of what to expect. Of course, since you're someone who doesn't accept payments upfront, you have the freedom to take as long as you please.