Unfortunately, asking how often to wax is a lot like asking how often you should wash your car. Like someone stated above, it depends on your specific situation. There really can't be a universal rule.
Personally, my vehicles are garaged unless I'm at work, and they see average numbers of miles yearly. I keep them relatively clean via hand-washing. For wax, I have been using Klasse All in One and the Sealant Glaze. This stuff lasts me 6 months at most, so a 4-5 month interval would be ideal. I always use a clay bar before the wax.
When I bought my truck new, it had sat on the dealer's lot long enough that it needed to be clayed and waxed immediately. The paint felt like sandpaper.
For me, one of life's great pleasures is washing your car and feeling the wash towel slide across the paint like the paint was made of hot butter. Oooooooh yes.:dancin: