In
economics, the long run is the period of time during which all of the factors of production are variable in
quantity for any given firm - as opposed to the
short run.
The actually length of the long run, in terms of calendar time, will vary greatly from industry to industry.
As all factors are variable in the long run, if the firm is not coverally all its costs ( both fixed and variable) it can reduce the quantity employed of all four factors of production to zero and hence, leave the industry.