Whenever I'm online, I leave it in a tab, and whenever a new "1" pops up, I check it. So "all the time"
I do it every minute I have also got gmail notifier installed so it gives me an alarm if I receive a new mail
my lotus notes mail is always on during office time. for my personal mail its like every 2 hours and varies very time I am waiting for mails
Actually being related to Internet Marketing field, I have a separate tab for my mail box that is always open in my browser till I am online and doing work. But during the weekends, I also use my e-mail and check that in 2 days only for 2 hours during the whole day apart from the normal working hours.
Every single day, my outlook and hotmail. I also reply to emails instantly, if I am away from my computer I use my Blackberry.
my biz email is being delivered to my iPhone 4 via Lotus Traveler software. I get it on my phone before it gets to my laptop.