I check my work email a bit over once an hour if I'm not specifically expecting anything important. Usually I note that nothing has come in, so it's very quick. I categorize each thing that comes in, which takes a few seconds for each category. I do not have a large volume of mail.

For personal email, I leave Mail open, and take note of incoming message popups, and sometimes click to it by accident. When I end up in Mail, I quickly make sure I didn't miss anything urgent (which is rare but does happen) and ignore the rest until the evening, or at least lunch.