This is a very difficult question to answer as it all depends on a number of factors such as your washing regiment, climate, and cleaners used. There isn't really a solid answer, but if you want to keep it looking great, applying the product once a month would be more than enough. Or if you wanted to go longer, the easiest indicator for when to re-apply would be to when you start to see the beading going away when washing.
Just make sure not to over apply, this product is unlike any others on the market so very little is needed. Only use 1 spray per 2' x 2' section. There are no unnecessary additives such as coloring, fragrances or any other useless fillers, it's very concentrated so if you use too much it may lead to streaks.
Also if you didn't know already, this can be sprayed onto every surface on your vehicle from the paint, glass, plastic trim and rims. There are no limits to it! We've even had customers use it on their kitchen appliances, granite counter tops, even the kitchen sink!