Lots of good info to consider, thank you all for assisting. At least two people have asked how often the pumps run. That is difficult to quantify as it depends on the weather. But I shall try to give some kind of guesstimate.
Our ~85 acre property is below mean sea level and is bordered on the north and south by a split tidal creek; most of the property is undeveloped land. The entire property is contained within a perimeter levee. Any rain that falls on the property, plus the constant influx of ground water, must all be pumped off the property into the creek. We have a system of drain pipes and open ditches that carry all water to a single holding pond; our two pumps are located at that pond. If the pumps fail to operate, the property floods. The grade is essentially flat so there is no natural fall: the water moves to the pond by virtue of the pumps drawing the water level down. Because of the distances the water must flow to get to the holding pond, the pumps never operate in a continuous manner. They turn on (alternately), pump the water in the pond down, then turn off while the pond refills.
In the summer, the groundwater is able to evaporate at a greater rate than what flows into the property, so the pumps basically do not run at all during the dry season. They are manually cycled briefly on a periodic basis just to keep them exercised and prevent the shafts from seizing up. During the wet seasons, it depends on the amount of rainfall (and how saturated the ground is), but the pumps can cycle continuously throughout the day & night, running as frequently as 5/5 (5 minutes on, 5 minutes off). I try to minimize the frequency of cycling by raising the "On" float during the wet season. And I've been pushing the property owners to widen/deepen the trenches and replace undersized drain pipe (with some success), to improve the flow of water into the pond. This is also why I originally wanted to explore putting VFDs on our pumps: so we could reduce pump output to more closely match the speed at which water flows into the pond. Running the pumps continuously during heavy rain, or on longer "On" cycles during light rain, should greatly improve their longevity over frequent short on/off cycles.