A satellite should have a planned lifetime, "expected lifetime", or design lifetime, but it's a balance between a number of factors.
Gunter's Space Pages is a great place to read about satellites and get a better idea what they are for and any special information about them. Here is the page for JCSAT-5 and JCSAT-6. Take a look there first.
This satellite is not in a low earth orbit (LEO) so there is no risk of the orbit decaying due to atmospheric friction. It will stay "up there" for a very long time, although not necessarily in the exact location it is supposed to be.
Satellites - including telecommunications satellites like this one - have usually been quite expensive to build and put into orbit, so in this example, if you can't reasonably expect it to keep working for 12 years, then it may not be economically worth paying for it to be 1) designed, 2) built, 3) application for and receiving a "spot" in the geostationary space obtained, 4) launched, and 5) maintained by ground control. Business plans to use this telecommunications bandwidth also rely on it's stability.
There is a huge amount of experience with telecom satellite orbital station keeping to maintain them in their assigned geostationary orbit position and pointing in the correct direction - attitude control. The satellite usually has more propellant than it is expected to need during its expected lifetime, because if it lasts longer, you can usually or at least often make more money maintaining it and selling the bandwidth.
But there is an expense to maintaining a satellite, so if you are not making money, sometimes you change to an inactive mode so that you don't have to pay nearly as much for regular ground communication bandwidth and control.