Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Anytime before the next iPhone is announced would be good enough. Your last best bet is when the Apple event is announced and the rumors are everywhere about a new iPhone. Even then you are taking a chance that the event may not be related to the iPhone. ;)

It's just once the phone is announced (officially) that the prices tend to drop as everyone wants out to get the next version.
 
I'm listing my 4S for sale next week. I kept my iPhone 4, for two reasons.

One, as a spare just in case since it was already setup to my liking.

Two, to switch back to so as to put my 4S on the market in advance of any announcements.

I've been following the used prices very closely over the last two refresh cycles. It seems that prices decline rapidly upon announcement of the next model. Whereas the pricing prior to announcement remain very robust.

That's my take on it.
 
You need to sell it before iPhone 5 (or whatsoever it is going to be called) is announced. The moment it is announced, price of iPhone 4S at Apple will be lowered down. Therefore, one to two months is your best bet.

However, take the matter this way: you are going to live without an iPhone for one or two months. Is it worth doing so for a mere extra few cash?
 
We've always waited until we got our next version phones and have always gotten the same or more than what we paid for the phones we sold.
 
depends on where you sell it. most peeps on ebay don't realize new products are coming out for atleast a few weeks. I sold my 16gb iPad 1 after i bought my iPad 3 and sold it on ebay for $300.
 
Just play it safe and wait unil the event gets announced. There are PLENTY of people out there have no idea about the press event. I've sold every one of my iDevices after the actual event and haven't lost a dime. I'd say get rid of it in the first week after the announcement though. As time goes by, the price will start to drop.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.