I usually don't worry about it at all. If the guy turns up a week later, I'll just buy it. I realize this won't work for many, but it has worked out fine for me.
If you can't imagine someone who doesn't log onto arfcom in two days, you don't have a very good imagination. It happens all the time, either someone gets busy, there is an emergency, or there are these weirdos who actually have stuff going on out in the real world and they don't worship at the alter of the boltface.
That said, most of the time, if you don't hear back from the guy, it's because he already sold it and can't be bothered to reply, or he thinks he has it sold and thinks its smarter to string you along than be honest about the situation.
48 hours is a good rule of thumb. Nothing is set in stone, mods/staff look at every situation on it's own merits (or lack thereof). Set your own standard, but cover your own 6.