Is there a specific definition of short run and long run? Like normally how long can be said as long run? And does specific time length of these words matters?
The actual length of time does not matter. Simply, in the short-run capital is fixed and labor is variable. In the long-run both capital and labor are variable (which gives firms enough time to, for example, build a new factory to match an increase in demand). It would be really fortunate if there was a general time frame, but that would vary from industry to industry. The long-run for, let say, Ford is a pretty long-time because they are very capital intensive. However the long-run for, lets say, a Clown Rental Service, is very short because it is entirely labor dependent. 

Hopefully this answer helps.