  15 years was widely held to be the standard during the diesel locomotive's 1st generation with switchers perhaps 5 more years. Of course GP7's and such soon proved that 15 years was an underestimation for well maintained quality freight power that still had useful assignments they were suited for. I imagine that 30 years is more like it these days although 15 years is still the length of lease for most leases and many locomotives are returned upon expiration.   For when you have the money, that's still the gold standard where passenger carriers are concerned. Look at Go Transit for example or Amtrak's plans/wish list for the future in regards to retirement dates that they've published in recent years as they try to develop a strategy for future fleet acquisitions.