What is the life expectancy of a GFCI receptacle?
GFCI longevity is mostly decided by two competing factors: 


Whether it's indoors or outdoors.  The elements will shred anything, and feeble little covers won't help. You have active condensation going on inside that device, it's going to have problems. 
Build quality.  Yes, quality matters.  A lot.  The corners cut by the cheapies mainly attack the longevity of the device.  Obviously it has to make it through its warranty period!  


If you have a GFCI on every outdoor receptacle, make sure you're not doing the common mistake of putting more than one GFCI on the same circuit, as this is at best wasteful and at worst dangerous.

Since GFCIs can feed downstream receptacles, the best plan is to have an indoor GFCI protect outdoor receptacles.