Great answers Zombirella and sktb0007!
Being a mentor is about more than reviews too. A good mentor should also know how get involved with the community and know who to contact if there's an issue. Also, even though some people get up to the 6 rank rather quickly, it's not always about time on site. If you've only been around a couple months, you're probably not going to be approved to be a mentor your first time around.