If you're using a quality synthetic such as Klasse AIO, Menzerna FMJ, etc - your protection should last between 4 and 6 months. The Klasse SG may extend that out as long as a year.
If you're using a carnauba-based product, count on about 2 months of protection.
If you're looking for a synthetic that's easier to remove than the Klasse SG, has great durability and also shines just about as well as a quality carnauba, you should check out
Menzerna FMJ (Froogle link) - wow.