I'll answer the bonus part since I already answered the others :P.

I think simpler stuff like sketches and the like should be prioritized since they can be taken care of relatively quickly but if you feel particularly inspired about one piece, you should stick with it since you're always better and faster when inspired. Also, drawing for yourself in between commissions does work; after all it wouldn't be fun to be all business and not get some time for yourself every now and then, you might burn out and get sick of drawing altogether.

I'll answer the first one from my own experience.

Once I bought a $10 sketch/doodle from teenycom on LJ a year ago. The woman would send me messages every once in a while by email letting me know that she was in the middle of working on it and would finish it eventually. After about 3-4 months I started to get impatient. Then she said she was unable to finish the art because she was too busy or something, and she would refund everyone's money. It still took her over a month to send the refund, and when I would inquire during that time she would say "I don't have the money right now, I'll get to you when I get paid next." Which showed me that she does what some artists do is take your money to pay off some immediate bill or something, and then doesn't have the money anymore when you want a refund.

I think it depends on what kind of art it is. If it's a long, elaborate, complex piece, or something expensive, then the wait time may be longer. If it's a simple sketch or something, there should be less of a wait time. Obviously 4 months on a cheap doodle is excessive. And I think that art should be done in order, unless like what Ibuuyk said, where you have a sudden inspiration on some other piece, then quickly work on that piece so you don't forget.

Also, I remember buying this one piece for $50, and he did a quickie sketch in a day. And I told him I was disappointed because I wanted something a little more elaborate for $50 rather than some 10-20 minute thing, so he agreed to redo it. It took him 3-4 months to finish, and I know he set mine aside several times to get to other customers, which irritated me a bit. But the end result was a whole lot better and worth the wait. In that case, if you're redoing someone's piece you should probably prioritize that, imo.

It's my opinion that when one looks to get a commission they should accept an expect that different artists have different methods. If said artist doesn't have a method you agree with, than choose some one else. It's pretty simple. You can't expect the world to fall in line with how you want it to be. Once you make the "contract" with the artist that's that.

What ever choice the artists make needs to be conveyed to their consumers; whether that's they prioritize smaller projects first, or what's easier or if they choose to complete bigger projects first to get them out of the way faster, or if they choose to go down a queue line.

Either way, what ever happens there needs to be communication between the artist and the consumer an an agreement, as well as some trust. Though personally I accept payment for completing pictures, not before hand unless it is entirely necessary, in which case I would explain the situation and if someone asks for a picture, they would be aware of the need and wouldn't ask for a refund any time soon.

When it comes to drawing personal things in between commissions, once again that involves communication between consumer and artist, and depends on the artist. Sometimes it's good to break away from something that feels like "work" and actually relax by doing something else. Just make sure to convey this to your consumers.

Communication done properly can help avoid so much drama and trouble.

When commissioning, I always abide by what I view as the freelancer's triangle: time, quality, cost.

The idea that if you want something fast and cheap, you're going to sacrifice quality. If you want something fast with great quality, the thing you're buying is going to cost you more. And so on and so forth.

Generally I feel like commissioned work ought to take roughly 30-40 days to complete unless there's been an agreed upon delivery date that has been negotiated between the two parties involved. If an artist promises work for a client at a specific date, then they should do their best to be within one or two days of that date at worst.

As for queue, it's really up to the artist's discretion who gets their work done first. Sometimes some commission work, by nature of what's being asked, isn't going to require as much work done. For example, a sketch commission versus a cel-shaded image. People should understand that more complicated images are, by nature going to take more time.

As an artist, I feel you reserve the right to do as much personal work as you'd like. The nature of being an artist usually means you may have sudden inspiration that youd like to put down on paper or spend some time on, which should be celebrated as opposed to discouraged. As long as someone's work is done within that 30-40 day period I mentioned, there shouldn't be an issue. While you're offering your talent for public consumption, you're also not an automaton that just draws on demand.

Hope that helps.

(hope none of this comes out wrong, sometimes have hard time saying what I am thinking)

It depends on what the artist says how long it will take them. Then that time shouldn't be like set in stone.

It can be flexible a little. This info the artist should made known about a approximate time for completion.

Like in a ToS. Or something agreed with.

If that artist has lots of work, school, etc, that should be made known in some way, that the artist may be

a while and no real set date can be made. Tho updates they have not forgotten can be made too.

As for a Queue, it I think it is generally polite to do them in order received. Tho I can understand

going out of order for things like you said, one piece may really inspire the artist, or one may be much faster,

or maybe a commissioner has not paid or something. There can be several reasons. Just make it known,

that may be possible so others are not upset.

Drawing for oneself I think is great. Sometimes a artist just has to take a break of doing others things,

and do something for themselves.

Like the complex piece you said, taking a small break with doodles may be something help one to be

at his/her best when they start it again.

All in all good communication is key, as long as its not over done. Which is almost always the commissioner.