An industry contact is someone at the particular job who already knows your work. Assuming that you have done good work in the past, it is *much* easier to get a job in a group with a former colleague than it is to get a job by sending your resume to strangers.
(I should point out that I'm coming at this from a computer industry perspective. As we've established in other threads, computer people flit around various jobs like bees about flowers... physicists evidently have more of a tendency to stay put.)
As for time, it varies widely. Sometimes people are ready for you to start tomorrow, sometimes you are the first person through the door and they want to do a thorough search before hiring anyone.