How long of a wait

I sent an e-mail to someone at 0803 today saying that I'd like to buy an item 'for sale'. I didn't ask them to budge on the price and I left my cell number. And I haven't heard from or gotten an e-mail.

How long do you guys/gals think is long enough before you just send an e-mail saying thanks, but, no thanks.

24 hrs.
Be sure to send it more than once and check-and-recheck the sendee's email address. The internet isn't rocket science but is IS fallible...
Steve not everyone checks mail every day & some are only online once a day, very early, very late etc. Unless you're in a big hurry & have found what you want elsewhere at a comparable price, I suggest waiting a day, or three.
I can't always check the e-mail box I use for this site every day and even then I have sometimes had problems with my e-mail software not sending replies promptly. Therefore, I think 24-48 hours would be a reasonable time period.
I agree with the above suggestions. If you haven't heard anything after 24 hours, send another email to confirm your interest and just to make sure that the message gets through. If you haven't heard anything after 3 days, consider it gone. You never know though, as i've had people get back to me up to two weeks later. Some people simply don't use the computer very often. Sean
I'll echo the other responses and note that alot of folks take a business trip and need to wait to get emails when they get back home. Others may simply be on vacation or a long weekend somewhere.

Damn, patience dude! People have lives you know, they don't camp by the computer all day. Huh? OK, maybe some people do. Seriously, I know how it can be when you see something you gotta have, but don't hear back right away, it's happened to me a lot. But that doesn't make the seller a flake either. Try to be patient and keep your options open, look for another deal if available.
Give it at least a few days but fire off a few more e-mails reaffirming your interest. When you really want an item and don't hear from a seller right away it can cause real agony (like frantically checking your e-mail or sending yourself e-mails just to make sure your e-mail is working properly), especially if there are other good deals popping up in the interim. However, like Sean, I have had sellers surprise me by returning e-mails up to a week later.

That being said, if an item has in fact been sold, a seller should mark it as such or simply send a short e-mail indicating that it's no longer available, thereby sparing interested parties the agony.
Patience, young Jedi! Generally one expects quick response, but it does happen that the ridiculous happens. About a month ago I was on a business trip and, on the way home, I had in succession a cancelled flight, a long-delayed flight to a different airport in which we missed the noise curfew and went to YET ANOTHER airport, followed by a cancelled flight and finally the flight home. All told it took 25 hours to get from the Los Angeles area to the Washington DC area. It was around 40 hours between email checks. Had you sent ME an email, I might not have gotten back to you within two and a half days.

Or I might have been overseas, where connections are often very, very hit-or-miss.