In most cases, Lithium ion batteries optimum life cycle is ~500, but even after ~500 charge cycles still have their ~80% capacity left, but at ~80% capacity, they work sufficiently good for basic tasks and standby, but in demanding tasks, they suffer and drain very fast. Your Xperia maybe the same, but taking good care of stuff prolongs their lifespan, so that could be one of the reasons in your case.
Manufacturers usually recommend replacing them when they reach their 80% capacity (~500), but since we are Pakistanis, so we keep using them till they die, just like we add water in empty shampoo bottle to take out remaining shampoo. 
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I have a Sony Ericsson phone bought in 2007 and still on original battery. It lasts for a day or even two days in standby and but in demanding tasks, it drains battery very fast, as battery is weak. It has li polymer battery btw.
