How long should electric furnace element last
If losing elements every year I would be concerned that overheat safetys are not functioning or the system is heating without air flow this is one thing that will shorten the life of the elements. 
Looking at the parts breakdown for your unit there are limits on Each rack, my question would be is the company troubleshooting to root cause or just pulling the cold rack. The thermal safety switches would be much more likely to fail than these elements and be way cheaper to replace. Things that cause the safetys to fail, dirty air filters limiting the air flow causing them to cycle open cool and then close, these switches are reliable to prevent overheats but continual cycling will wear them out. 
How long should the heating elements last ? I would say at least 20 years with proper air flow, I have seen them closer to 40 years old in several cases.
I would suggest that you make sure to change your air filter and make sure it is the type specific for your unit, a super high filter may reduce the air flow and cause early problems. Make sure your registers are open closing two many down can drop airflow and cause the same problems as dirty air filters. Last I would check the blower motor if belt driven it may be slipping, but at its age it probably needs oil in the bearings (every year) just a few drops usually there are ports at each end of the motor for this. Last next time a element fails I would want to test the element itself and the thermal safety with an ohm meter (power to the furnace turned off) the resistance on the safety should be very close to zero ohms, the resistance on the element will probably be 20 ohms or less I am not sure because the part number did not show the wattage, but the resistance will be below 100 ohms a bad element or safety will usually show open.these are the things I would check now for proper flow and later when one has failed you may discover they are replacing the rack where a $10 part is all that is needed.