I've used Rejex for years with good results. On paint, it seems to last about 3 months depending on conditions, use of a garage, etc. 4 months of protection is certainly feasible. I usually apply a double coat a few hours apart and it is easy to apply/remove. It's a full synthetic protectant and contains no wax, so it won't stain plastic trim. And let it cure like the instructions say before taking it out into wet weather.
Optimally, Rejex should be applied to clean paint. This is a quote from their website: "While it can be applied over existing wax, the duration of protection will be reduced as the softer underlying wax degrades. There is no need to wax over RejeX—wax won't adhere to its shinier finish. RejeX will not bond to automotive products that contain silicone."
Although I do think that these petroleum based synthetics act as a mild solvent that will remove most of the old wax when you apply it (just my non-scientific experience.)
Now that I switched to Wolfgang, I mostly use Rejex on wheels to protect from brake dust. IME, wheels should be protected with full synthetics and not carnauba wax (gets sticky when hot, which isn't good for keeping brake dust off the wheels.)
