In my view, the most effective duty-standby arrangement has to be judged by the engineer. The question to ask is, what is the dominant failure mode. If it is a random failure mode that is dominating, there is very low risk of increase of failure for switching them every fixed period of time. If the dominant failure mode is in the realm of wear, then one might want to consider off-setting the operational hours so they don't have a perfect storm resulting in plant downtime. If the dominant failure mode is false brinelling, one might want to consider running all the redundant equipment at partial load instead!