I would say that 4L of developer for a 16x20 is a minimum if you are running fiber paper, at least that's the lowest I've been able to do without having to constantly push the print down for the first minute. In terms of usage, I normally set up the darkroom here at school with 6L of chemicals across the board. The chemicals can usually last through a couple days of printing, I'd like to think that the equivalent of 100+ 8x10's are run before I have to change chemistry. The only thing is we use Marathon chemistry, which is supposedly formulated for longevity in a high use atmosphere.