I've been a professional prop maker for 6 years now and I usually respond to emails in less than 24 hours. It's just how I think business should be done IMO. I've had many customers come to me after having dealt with a deadbeat commissioner. For many commissioners, this is a hobby... And they treat it that way. So as a customer, consider how important your project is to the person making it. Because this is business to me that I hope will one day replace my day job entirely, I take it very seriously and have even taken time off work unpaid at times to meet someones deadline.
On the flipside though, I get frustrated when a potential customer gets to the point where they're all but ready to pay for a commission something and I never hear from them again. I'm very busy and have to constantly calculate and schedule out deadlines, supplies needed, etc. I'd rather have someone say they changed their mind instead of leaving me waiting for their payment to start on their project. Just sayin... There's give and take on both sides.