Depending on the environment you drive in and your vehicle cleaning and maintenance routine, RejeX should be able to shine and bead water for well over a year.
RejeX will "reject" dust, bug splats, brake dust, etc., until the coating starts getting worn through. On a typical 10,000 mile per year daily driver in a dusty, industrial area, I'd estimate 3-6 months depending on how picky you are. If you want bugs a brake dust just to fall off with a water hose blast, it's 3-4 months or so. If you're talking about being able to wipe off but splats mostly with water pressure and being able to get the last few cling-ons off with a wet rag, it's more like 6 months.
Most people who use RejeX end up building up a few layers because it doesn't break down like typical waxes do, so you end up getting a few layers on the vehicle over time. It will last months, not days.
A lot of guys can't seem to wrap their heads around the idea that using RejeX is, in fact, that simple. IMHO, you don't need to complicate matters with fancy applicators or trying to top it off with wax. If you feel compelled to do more work, you'd be better served by just putting on a subsequent coat of RejeX.