Most sump pumps run for 8-12 seconds and are rated for 6 starts an hour. They can handle more during heavy rain conditions, but 6 starts an hour is the norm. Two things could be happening. See if you have a check valve in the discharge line. A check valve prevents water from running back into the sump pit once the pump shuts off. OR You could be on an under ground stream with a high water table. If this is the case, an active sump pit is not uncommon. I recommend installing an additional sump pump, if the pit size will allow. This will give you double protection in the event of a failure. Also I recommend a high water alarm or battery backup system. Check out http://www.zoeller.com for details.