Expect about 30 to 40 charges on average before they most likely need replacing.
The Turnigy NanoTech 750 mah battery seems to be the go to battery for longer flights and a little more power on the x5 series, as they have a higher C rating than the stock battery.
The one thing however to watch for is the plug on the nanotech batteries...they dont match up to the stock x5c plug and need to be removed and reversed to properly fit..Failure to do this will result in frying your flight board.
The polarity is correct but not how it fits to the x5c plug..the battery plug needs to be flipped around.
EDIT: Batteries are hit and miss..all of them are mass produced and there are a small percentage of "bad" cells that can get purchased.
Always charge your battery completely charged before its first useage..Never store a battery that is fully charged..For a lipo, 3.7v is the storing voltage one should be stored at.
Many of these toy USB chargers really destroy batteries as they arent considered "smart chargers"...Many of them will not turn off at the correct voltage and overcharge your battery before it shuts down, which most certainly will kill the life of the battery prematurely...a 1s lipo should never be charged above 4.23v maximum.
A good battery charger is a very wise long term investment to ensure proper charging and longer battery life of all batteries you may use.
The Imax B6 seems to be a very popular charger among toy quad owners.
Beware of ebay and amazon imax b6 fakes...they are not as superior to the original.