How long the distance covered?

The achievable distance is not specified by Sony. This might sound strange, but keep in mind that if a range was specified, that would be in ideal conditions with no other signals or obstructions. In real-life situations this is never the case so actual range will always be different and a specified distance might give false expectations.

The transmitter has 2 power output levels (CE is applicable for this model); 

30 mW/5 mW selectable (UC, U, CE, LA, CN models)
10 mW/2 mW selectable (J, E, KR models)

By default you'll work on the lowest setting, in this case 5mW. This will give the best battery life.

For demanding situations you can switch to 30mW, but this will eat through batteries much quicker, so better not to use it by default.

These audio transmitter sets are designed for typical camera shooting conditions and you can expect it to perform very well. However, we often speak customers who suddenly experience problems when working in the field. In this case there are other signals or obstructions which interfere with the signal. In this case you'll either need to find a clean frequency band and/or increase the output level. By doing so you'll be able to solve most problems, but keep in mind that wireless transmission in the end is not 100% guaranteed to work in every situation.