NEPA routinely contacts 15 editors on a first submission and if they are interested, we send out the proposal to each of them. Each time a rejection letter comes back, we send it on to the author at the end of the week. The final sale should take about 2 - 3 months, but I have sold projects two or three years after they were written, and seen them be successful books. Editors change houses a lot and when they do will need to build up lists quickly, so I submit old projects to them at that time. New publishers when they take over a house frequently change its direction and can buy in categories they ignored before.
Companies merge and take on new interests.
Markets change within a few years, creating a demand for a book that was unsalable a year or two earlier. So don't give up. This is what we must do, as writers and agents, in order to that our work see the light of day.