How Long Should a Microwave Last?

We bought a GE microwave in Feb 2012 from Home Depot.  A with a trim kit to fit into the cabinet above our oven.  It cost us $515.71.

Starting yesterday it started cooking at less than 5% power.  I put in 3 cups of water and a cup of grits.  After about 9 minutes of cooking, the the water was tepid.  I put in about a cup and a half of water, after 9 minutes I could barley tell it was warmer than after I got it out of the tap.

Anyway, we did not buy an extended warranty.  We rarely to never buy them.  It is like buying insurance for your purchases.  It certainly would have been worth it in this case, but over the averages of all purchases of electronics and appliances, I am sure it has been a better deal to skip them.

I am not sure if this defect is common or if it was a fluke.  It looks like we are shopping for a new microwave.

The model number is JEB1860DM2BB.  Here is the manual.  Here is a similar model at Home Depot.