Hey
1)Yes, you can't control how much and when you will bleed. Everyone is different so never get worried if you're not the same as a friend.
2) That's fine, most people change less than that, but depending on how heavy your flow is you may need to change more or less frequently.
If you need more info try http://gurl.com
It's great for this kind of information! Try these pages:
http://www.gurl.com/findout/fastfacts/articles/0,,675750,00.html
http://beinggirl.com