You can't really polish (wax) your car too much, every 2 months is good, every month is even better. Don't leave it longer than 3 months if it's in the sun a lot.
BTW, you really mean wax it, as polishing is taking a fine layer of paint off every time.
If paint is well looked after, that is waxed often enough, you won't have to polish it.
Only polish if the paint has lost it's shine.
Polish also 'cuts' the paint slightly, otherwise it wouldn't be called polish.
There are many different grades however, and depending on how bad the paint is, determines what grade polish to use.
Always finish with a wax.
Can't wax a car too much no such thing. Usually for newer cars a fine polish is whats needed as it cuts alot of the small stone chips and excess touchup paint (small white bits on the bonnet that i really really hate) out but doesn't takeaway the paint. After a polish the surface should be waxed to seal the paint otherwise the paint will be due for a polish again.