Mine get waxed once every 2-3 years whether it needs it or not.
On a truck that see's a lot of trail time it's just going to get scratched all to hell and back so once it gets to where I can no longer stand it I do the following.
Wash it thoroughly to remove all the dirt/contaminants from the paints surface.
Clay Bar all the painted surfaces to further remove dirt/contaminants. If you've never done this it takes a lot of time but you will not believe the results.
Get out the Porter Cable 7424 XP and some Meguiars Ultimate Compound and buff/polish all the paint surfaces. I'm cheating a bit here combining two steps with one compound but the paint will never come all the way back from that many scratches so a complete paint correction is a waste of time for me.
Put on a coat of Meguiars Ultimate Paste Wax.
The entire process listed above will take me 12 hours to complete but even as beat as the paint on my truck is I can still manage to get it looking decent in the end just takes a lot of work.