Is there a specific definition of short run and long run?
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.