You're right, a whole day might be a tad ambitious!
I have two wooden ones, and I can only wear them for an hour and a half at a time. I started out wearing them a half hour at a time, and gradually worked my way up to an hour and a half. I write, so I spend a lot of time sitting in front of my computer; that's a great time to put them on. It's also a pretty good test of my concentration.
I added weights to them about a year ago; I am presently wearing five one ounce fishing weights on each nipple to elongate and stretch them. I had a problem with inverted nipples when I had my kids, this made me unable to nurse (not that they wanted my tits anyway; both my boys wated their bottles, not me.) I love the pull; but with the weights on I can just barely make it to 45 minutes with them on. Then I take them off for five minutes, and put them back on for 30. It hurts like hell, I agree with you on that...but it's also a delicious, tingly kind of hurt! And I also wear them when I masturbate, when I'm standing in the shower (the swinging weights yank my nipples in all directions, and that hurts! I can cum just from that sometimes!