I don't exactly "check" email. I have Outlook permanently open on my secondary monitor and it downloads new mail every five minutes. However, I really don't look at it except about once every four hours or so (I use email constantly for work, so it wouldn't be safe to make the frequency lower than this). At the four hour point, I flag every item in the Inbox that requires some kind of non-immediate activity or response (and respond to the ones requiring immediate response). The flag is automatically set for tomorrow with a reminder, which effectively will give me a to-do list tomorrow from today's email. After flagging the appropriate items, I select all the emails and file them automatically and all at once with the batch file function from SimplyFile. The whole process (flagging and filing, thereby emptying the inbox) takes about one to three minutes for each four hour cycle.
How frequently do you think you *should* check email? You can also say how frequently you do in comments.
Personally I'm sold on thinking you should check it around once a day, not necessarily without fail. That increases focus on both email and non-email, and minimises getting sucked into distractions. But some people I know disagree. Some believe in getting notifications whenever a new email comes in.
For anyone who'd like to check email less often and uses GMail, I recommend using http://inboxpause.com/ and this full screen compose link: https://mail.google.com/mail/u/0/?ui=2&view=cm&fs=1&tf=1&shva=1
Edited to add: I'd recommend everyone at least try checking only once a day, at least for a few days, to see if you find it more productive and/or relaxing. That'd be a big enough win to make experimenting worthwhile.