For some.. we get paid just for you viewing them. Others, you need to click em. I don't think you need to spend a certain amount of time. Just click em.
The second question, the cookies are to keep track of what ads you've seen and when. I know for some of the fastclick stuff, you see them only 1 per session or 1 per day. If you blow away the cookies, its probably just going to keep showing you ads.
As Matt said, I wouldn't worry about it. As it is, the ad market is in the crapper. The number of visitors we have increases every month, but the last few months our ad payments have gone down. Go figure.
"Blu-ray is just a bag of hurt." - Steve Jobs