Pump Operation Standby Cycles

[mflam]

I'm going to ask this question a couple ways.
In a wastewater pump station, how long should a standby or spare pump be allowed to stand without operating?
Or, at what interval should should the lead-lag pumps be switched?

 

[bimr]

The pumps are set up in a lead/lag arrangement with automatic alternation after each pump cycle to balance run times an minimize starts per hour.