Dive into your creative stream
Honestly, it really depends. Of course, as an artist, I understand people have lives outside their art and the internet. Not to mention others' commissions to worry about while doing mine. On the other hand, if I paid them before they finished, I would be nervous if they took weeks/months to even get started. The people I commissioned usually were super fast and finished sooner than expected so I haven't had a long waiting experience yet. I think I only ever waited super long or never got a certain thing when I asked for requests or made suggestions. That's definitely when I'm more lenient with wait time because I'm not even paying them. Yeah, it's sucks to see an old request fade into obscurity but at least I didn't lose money over it.
The key to a long waiting period between an artist/customer relationship is communication. I'm more likely to wait longer without too much badgering if the artist is transparent about the likely waiting period. They don't owe you their life story, but it's nice to have an estimate so you don't spend all your time wondering. I think showing the progress with a watermark ( a pattern of watermarks if you want to be safe XD) during the waiting period could help to ease the customer's mind too. It might even help the customer catch something that can be changed easily in the early stage. I think I probably would pick a week but I honestly would be willing to wait longer if the artist gives a rough estimate on likely when things will be done. They don't need to explain why but it would make me feel good the artist isn't leaving me in the dark. Though the artists I commissioned I don't remember ever making me wait longer than a week (unless I count plushie commissions which are a completely different beast to digital art). TLDR Customers be patient with artists because they are only human. Artists and customers, respectfully communicate with each other during the process so no one is anxious or feeling duped for one reason or another.
I've been wondering about this for a while, but when you commission an artist, how long would you want your piece to be done? Blink once if a day, blink twice if two days, blin-
Just kidding, I made a poll :D