Joel, the short answer to how often is... it depends. Type and quality of the wax, environmental conditions, and level of prep work all determine how long the protection will last. ASSuming that you washed with auto soap, clayed, and used a GOOD carnauba based paste wax, I would say that if left outside, you'd be good for about 2 - 3 months. Soap and wax in one bottle? 2 weeks at MOST. Paint sealants last longer, but lack depth and "warmth".
Is layering (AKA multiple coats in a short period of time) beneficial? Depends on who you ask. It's split down the middle at 50/50. Even the advocates of layering agree that 2-3 layers is the max. More than that and you are just wasting both time and product. The critics argue that once the pores are filled, that's it. My opinion? If one coat is good for 2 months, then I just wax once a month...... But remember that my name is on the side of my car too.