(commentary)
The whole rumor about an impending D610 is causing a re-emergence of the number one misconception in photography.
First, the rumor: that Nikon will introduce before the end of October a D610 model to replace the D600 model. The cynical extension of the rumor: that this is being done to "relaunch" the D600 because it isn't selling, and that the camera isn't selling because everyone thinks it has an oil/dust problem. This cynic's view basically also requires that you believe that a D610 wouldn't differ much from a D600. Maybe just stick in the 51-position AF system or take out the AA filter, for instance.
So what's the misconception that's re-emerged? That cameras are investments are supposed to hold value.
Heck, we don't even need a D610 introduction to disprove that. Refurbished models are going for US$1600. Technically, a US$500 discount from list price with more warranty (90 days) and in better shape than your now used D600. Thus, even the most lightly used D600 bought at list almost certainly has lost 24% or more of its value. Some investment that is.
Of course, some of you bought the D600 when it was basically US$1800 with the kit lens during Nikon's insane Christmas offer last year*. For you, your "investment" has only lost 11% or more of its value. Still not a portfolio I want to buy into.
If you really think cameras are investments, then I need you to develop a system of Put Options I can buy. Let's see, I'll offer to sell you a used D600 for US$1800 in three months, how's that? I'd pay something for that Put.
A camera is a money maker only for pros, and then only if the pro is watching incomes and outgoes really carefully (and depreciating the asset as a business expense). For everyone else, it's just another thing you bought because you thought you could get pleasure from it. Yes, it has some residual value, but realistically the actual formula should be PRICE PAID => PLEASURE + RESIDUAL VALUE. Since we know RESIDUAL VALUE is going to decline fast, and for a mass-produced consumer item like the D600 it will basically continue to decline, your PLEASURE component had better be very, very high.
So the answer to the question in today's headline is simple: as much pleasure as you derive from it. Consider the residual value, if any, a bonus.
*Why was Nikon's bundle offer with the 24-85mm kit lens "insane"? Because it effectively took a new camera and valued it lower than the usual end-of-life price. If Nikon wants to know why the D600 isn't selling, this is part of the reason: people don't think it's worth US$2100 because Nikon effectively said it wasn't almost immediately after release. Lower the price to something less than the implied price in the insane bundle sale and it might start selling again. Of course, the other reason why the D600 isn't selling is because of the debris/lubricant problems that have been reported, and the fact that it's pretty easy to figure out that Nikon is replacing a lot of D600 shutters when these cameras come in for cleaning. Effectively, Nikon's insistence on denying (or rather: not acknowledging) that there was a problem with some D600 models means that everyone believes that all D600 bodies have the problem. In short, Nikon has totally botched the D600 intro. It wasn't the D700 replacement people wanted, they sent hugely mixed signals on how much the camera was worth via that bundle deal, and then there's the shutter problem to worry about.
