which factor is responsible for lipo self life, charging cycle or time at using properly?
if I charge lipo battery at 50 % and keep it for two or three year unused, will it discharge it self or hold same power when it was charged?. How long a lipo can hold power without use?

Charge/ discharge rate plays a major role.Lowering the value of these will increase it's shelf life.
Depends upon quality of the cells.
Approximately, a voltage drop of 5-8% might be seen if left unsused for a period of time.

Capacity will definitely decrease. For least degradation in capacity, it's best to store at 3.85 V and at cool temperatures.

The batteries you buy at stores are likely years old.

Battery degradation is a function of charging rate, discharging rate, time, storage voltage, storage temperature, number of cycles...

Took out 4 small 2s lipos not used in six years!. Stored at around 3.85v per cell.
Nine Eagles 180 & 250 mah, two Zippy 350s.
All still showing between 3.78 and 3.85v per cell.
Of course voltage alone is not a sufficient indicator, the ability to provide enough current also needs to be checked.
I was surprised that all four are still able to take full throttle on an Emax 2805 with 6x3 prop without sagging.

(On the other hand, a brand new Turnigy 2200mah 3s, left fully charged, bloated up in 2 months. Still shows 3.80/3.82/3.84 but is no longer able to supply useful current, ie, sags badly after 1/4 throttle. Of no use in a model. Perhaps I'll put it in a Tx!)

Regards