It keeps....damn near forever! This summer I pulled my 424 stove out of storage and fired it up for the first time in nearly 20 years. I didn't add to or change the fuel, just used the half tank that had been there all this time.
I've picked up 4 stoves and two lanterns at thrift stores the last few years, only one stove had problems with the fuel having turned to varnish. I've no idea how old it is, but it could have been 30 years old. I think that it might be wise to store a stove/lantern full so as to leave as little space for the light distillates to evaporate into... or just use it often and it doesn't matter how it is stored. :-D
I picked up an old 288 lantern last week and it had some fuel it in when I got it of unknown age. In the store I pumped it up and tested that it didn't seem to leak anywhere and that the pump and valve worked. I took it home, pumped it up, noticed that the mantles were new and hadn't yet been burned, so I burned them and it fired pretty much right up. The fuel is noticeably old considering how it starts up, but topping off with fresh white gas will clear that up I expect.
The one thing that I've found to often be a problem and can be a real challenge to resolve is if the one-way check valve in the pump is gummed up. "The mission here is to keep pressure from leaving the fount but allowing it to get in. The check valve has a ball bearing in it that performs this function." If it sticks open, or is slow to close, it can allow pressurized air/fuel vapor to escape the pump inlet (what you cover with your thumb) which is smelly or potentially dangerous if it is being pumped while lit. I had one stove that had an issue with that, but I was able to dissolve the varnish/gum and clean it to fully functional by removing the pump piston/cover and pouring carb cleaner into it and letting it soak a few days. Technically the valve can be removed and replaced, but from what I've read and how it looked, it is difficult to remove if it is gummed up and likely to screw up the threads and such where it installs down in the base of the pump. Other than that, they are pretty flawless and rebuild-able.
If y'all haven't yet seen it, oldtowncoleman.com is a wealth of info about these old devices:
http://www.oldtowncoleman.com/tech/re2lanta.htm
Does anyone know where to get new seals for the valve on a stove tank? I have one that leaks liquid gas out around the shaft (near the handle that you turn) and it looks simple enough to disassemble, but I've been unable to find the part to install.
Oh, and regarding travel with a fueled stove that I didn't see anyone mention... I believe Coleman's instructions state that it is safe, but the stove should be depressurized before it is packed for travel. This is done by opening the fuel tank cap enough to vent the pressure. After that, it is safe to travel in a vehicle... the risk being that if the valve were bumped/turned while packed away or in a vehicle, it would start releasing pressurized gas vapor into the vehicle.
Cheers.