Basically if I don't delete it as soon as I receive it (ie. some newsletters), then it will be in my inbox forever. I keep a lot of emails so I can keep track of communication and use the search function to find them later. I have a small handful of folders for some sorting (mostly to keep my Twitter stats separate from my work emails or whatever) but mostly I just let my inbox fill up. It might be nice to start again at 0 some day but I'd rather waste my time doing other things that I actually care about.