I don't submit to any agency but I am surrounded by photographers that do and am friendly with a few editors for the majors. Now keep in mind that being in the New York City area, there are quite a few photographers all racing for a piece of the pie and the following is influenced by that. From what I hear, the magic formula these days is submit to the agency in no more than 2 hours after you have taken the photos (not the end of the event) or you are usually deep in the pile and unlikely to be picked up. That time limit increases if you have special access or if there are less than 3 photographers on site. Not an easy workflow to pull off but it seems to be the one that works.