When to replace a well pump?

We have a 300 foot deep well, about 2.5 gallons per minute. The pump is around 20 years old, minimum and I understand can't pump more than 5 gpm (some sort of limit because they guy burned out an earlier pump leaving the sprinklers on). If we water with a sprinkler for 20 minutes or so the pump keeps running for 20-30 minutes after the sprinkler is shut off.

Is this a function of the well refresh or is the pump getting old? What's the life expectancy of a submersible pump? Would it be prudent to replace it now, rather than wait for it to quite when the temperature is 25 below? ( We livein Montana)
Kent

Pumps always quit at the worst time. Replacing the pump now doesn't mean it won't still give you a problem when it is 25 below. That is just how Murphy rolls. The new pumps that are made today, are not as good as the ones made 20+ years ago. It is not good for it to take that long to fill the tank and shut off, after the sprinklers are off. I would guess that the sprinklers depleted the well and the pump has no more water to pump until the well recuperates. If the pump runs the sprinklers with good pressure, it is probably still working OK. New pump or old, a Dry well protector like the Cycle Sensor is probably a good idea. Your pump may be on it's last leg. However, it may last another 20 years. Only Murphy knows.