i feel like the time it takes to put a commission together depends on the topic, the free time of the artist, and predetermined expectations explained /before/ the artist even begins to make the piece. If no one has told the artist when it needs to be done, it should be expected that the amount of time it'll take to receive the piece may be unknown...

basically, unless you have an explicit deadline, it's at the artist's discretion. they'll do it when they have time/are emotionally able to put their effort towards the work.