In economics, "the long run" is the time horizon over which all inputs are variable. There are no fixed costs in the long run — you can change factories, labor forces, technologies.
The confusion: "long run" and "short run" are analytical categories, not fixed time periods. For a street vendor, "long run" might be a week. For a steel mill, decades.
Keynes: "In the long run we are all dead." The irony is that Keynes cared deeply about the long run — the aphorism was aimed at economists who used long-run equilibrium to dismiss short-run suffering.