Every once in a while I see assistant professor job postings that don't have set deadlines. Sometimes they would add that applications are reviewed on a rolling basis, or that "until the positions are filled". Usually how long are these review periods? Does it mean that the job is still open as long as they have not taken down the notice? I am aware that one possibility is that someone simply forgot to remove it from their web site.
One possibility to be aware of is that tenure track hires usually need to made after a national search. Yet some departments like to hire by going after "targets of opportunity". These are individuals already in faculty positions whom the department have identified as exceptionally talented and potentially willing to move. In order to facilitate this process, some departments keep an open search on the books for a protracted period of time.
The point is that some fraction of these rolling searches are probably not intending to hire from the pool of applications that come in unsolicited. That said, I also know of cases where the applications that come that way turn out be so strong that a department ends up hiring from that pool.