Are these motors sealed or vented? This post will contradict what i posted above but i have seen motors fail prematurely from heat. Normally in a kitchen grease gets into everything and why i would recommend a sealed motor. But, now we have to look at how hot this motor is getting too. If it is a sealed motor then you may be seeing the failure due to overheating/lack of cooling. I just went through this with a farm here outside of Town where the guy was burning up fan motors every yr. All of them were sealed and he insisted they had to be sealed for dust/moisture. I talked him into trying 2 new fans that were vented, it has been 2 yrs now and they are still going strong. Motors are very tough. I have seen them submerged in water, dried out and run fine. At the local garnet mine i saw motors run 24-7 for 20yrs covered in garnet dust/sand. We used to hit them with a rubber mallet when they slowed down so they would blow themselves out the dust build up inside. The dust would shoot out and the motor would pick up speed and keep right on a running. But anytime i have found a failed motor heat was usually the factor.
I would check to see if it is a sealed motor or not and check the temps while the kitchen is operating around the fan unit. I would expect 140-160 deg temps.