I'd give them 3-4 days to email me about it, but I wouldn't "expect" payment within a few hours, 1 day, or even 2 days. After 2 days, I start to worry slightly. After 3-4 days, I'm really hoping that he'll email me about it. After that, I'll wait impatiently, waiting for day 7.
Personally, I pay within 1 minute to 2 days. There have been days when I forgot that I had bid on an auction on eBay, or I didn't realize that my money wasn't in my bank account. If it's in my high-interest bank account (I get around 6.5% interest), I move it over to my "regular" bank account, which takes around 24 hours. Then I pay. I usually email the seller to tell him this, but some people don't do this.