Whenever you're getting information that is better explained in text than in person, and is person.
For instance, someone recently sent these 2 pieces of advice to me:
Sit on lap – Take her hand and move it above her head so that she spins around. Then, when her back is towards you, sit her down on your lap and hug her from behind. . Kissing with release – There are many good techniques for kissing a girl. One would be to put on some flavored chapstick and say, “You know what the best part about chapstick is? Here, smell it”. Let her smell it and then continue, “It not only smells like strawberries, it actually tastes like strawberries too. Check it out”. And then you go in and kiss her.
Worst mode of transmission ever!
People's expectation clock starts running from the time they hit send. More improtantly, deadlines related to the email content really sets the agenda for how often to check your email.
Then change people's expectations, including those of the deadlines appropriate for tasks communicated by emails that people may not see for a while! (Partly a tongue in cheek answer - I know this may no be feasible, and you make a fair point).
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.