Whenever gmail notifier says I have a message, generally. If that little icon is blue, but I know I don't care about the email that originally caused it, I probably check it after a few hours (unless I get another notification while I'm at the computer and want to read it).
Work email - first thing every morning, then have it set to check for new mail every 10 minutes.
School email - almost never. School email is mostly only useful to me for getting discounts on software and shit. Will check at beginning of semester and if I'm expecting something from a prof, otherwise I pretend it's not there.