If I'm going to be outside for most of the day I definitely re-apply because my super-pale skin will burn otherwise! Usually one application mid-day is good enough, though on my Hawaiian honeymoon I still got a little sunburned with twice daily applications, I guess since the sun is more direct at those lattitudes. If I'm outside all day and I'm not going to be getting wet (like at the beach or pool -- obviously I don't wear mu there), I just wear some basic eye mu and a light dusting of powder or mineral foundation which I can easily reapply after reapplying the sunscreen.
However, for a normal day, I don't even wear sunscreen (though my foundation has a little bit of SPF) .... I'm in my office all day long, and only go outside for a few minutes a day walking to and from my car, and walking to the company cafeteria for lunch -- I'm probably only outside for 10-15 minutes a day! And since you need 10-15 minutes of sunlight WITHOUT sunscreen a day so your body can produce vitamin D, I figure I don't need sunscreen! (Plus, vitamin D is very important to prevent osteoporisis, which I already have several risk factors for!)