I've never let it go for all that long because the paint always gets dirty/rained on and needs washed anyhow. But generally I'd say it'll last a few weeks or until the next wash.
If you're using the car in hot or otherwise extreme conditions it might not last more than a couple of weeks. If you use it in more gentle conditions it might last a month. Think of it as a very delicate/short-lived wax, less durable than Souveran, for instance.
Reapply after every wash.
