We have a 14 yr old daughter and we would not leave her home alone 8+ hours. Our daughter is very responsible and mature but we feel that 8 hours is too long. She stays home alone sometimes but not more than 4 hours.
I think 8 hours is a LONG day for an inquisitive child to be spending home alone.
I know people do it. I let my daughter visit a friend last summer thinking mom was there and later in the day I found out they were alone. Now, both kids are good kids and responsible but still....
I will leave my daughter home alone but it is usually not for more than 2-4 hours. She loves being home alone and I am glad she feels independent about it but I do limit the time.
As for security, we do have an alarm system and protective dogs. My daughter sleeps very soundly and will sleep until almost noon. I know from experience, if I am in the back part of my house, I do not hear someone at the door. I rely on my dogs to alert me....which they are VERY good at. Summertime is also a higher crime time (even small crime) because there are some bored teens who want to have fun at someone else's expense.
Bottom line....everyone is different and you just have to do what is right for your family.