I used to check my e-mails twice per day. I will not get registered with unnecessary sites so that it helps me to check only the mandatory mails thats how i make my work easy.
I check email several times a day and always have my mail client (Thunderbird) open when on the computer.
Not too much, unless im expecting a client or something. The other mails i receive are junk, or facebook notifications, so not really interesting .