One of the really nice things about Linux on z is the fact that the underlying hardware has a long life. This means that for complex software solutions that need extensive and expensive testing you can keep the system going over a long time period. The Linux distributions on the mainframe are supporting this model with their long term support contracts.
But how long can you get service? There is a now updated summary presentation out on TechDocs called "IBM Mainframe Life Cycle History" (PPT, PDF). It has a nice overview chart which gives the answer to this question: the average support cycle over the past 21 years has been 11.3 years!