This really depends on your child and his maturity.
My daughter at 9 was ok with me being away for 1/2 hour or so for errands, etc. We have increased that time depending on her behavior/responsibility up until present. Right now she is good to be alone (13 yrs) for a while but I still keep a lot of contact via cell phone and home phone. A rule is....if she does not answer either phone after reasonable attempt, (I know she gets caught up on phone with friends, wii, etc) I am coming after her with authorities. That has not failed me yet. She keeps both phones in same room to prevent that from happening.