Actually if your OS is still installed, it will never cease to be current...
The problem is if you want to REINSTALL it on an older box. I tried that not too long ago on a secondary machine. Wiped/reformatted/reinstalled, and had SO many problems getting XP to go on it wasn't funny. Not the least of which was you have to manually download and install the SPs until you get to SP2 or 3 to get auto updater to work. Gone are the days you can just set it to install all the updates you need... Those manually downloaded SP1 updates kept erroring out and being pests, corrupting entire OS installs when they quit halfway through, etc.. It wasn't fun in the least.
However, if the OS is still up to date and installed at least to the point you have windows update running, it should last a while.
Unless you need to reformat for some reason.
I ended up moving to Win7 for the advanced memory support and x64 bit features, but XP was still good while I had it. I did like it as an OS. There's a reason it was around for so long.