half a year, providing I get updates on the process.

I think if you have not heard a word at all, two weeks in, politely ask how the piece is coming, but be sure to be polite and thank them for their time and for doing your art.

I feel like I have a lot to work on before I can start another commission.

Usually a couple months. As long as the artist is very responsive and gives updates. If they're completely silent and don't respond to messages I'd grow impatient fast. I always try to be very responsive to people who commission me and respond with WIPs as I complete steps in the pic. Because that's how I like artists to communicate with me when I commission other artists myself. Haha. It might be strange but I still commission artists to do pics for me even though I draw hentai all the time.