Isn't it just a version of automatic gearbox fluid?
Unless it looks or smells different, e.g. due to overheating caused by a fault or abnormal use, I believe it has an indefinite life, though I had my current cat changed at ten years, and the filter. I haven't had many automatics, I think my previous one had some sort of repair and was refilled then.
Same with power steering, axles and manual boxes, I can only remember having new fluid or oil after a leak, repair, or replacement parts. I keep bottles on the shelf just in case and use it almost never.
edit
I just looked mine up on
http://www.shell.com/motorist/find-the-right-oil.html#iframe-L2diL2VuX0dCL2Jyb3dzZQ==
and it specifies the ATF, then for power steering it says "as for ATF." I did have to top it up to maintain the level while waiting for a leak on the pump to be fixed, hence I bought 500ml and still have most of it.