Don’t use the windshield washer variety unless you feel like pissing away your money. Its more expensive, doesn’t work as well, and washes off quickly, not to mention that you go through a bottle once every couple months. \
The original product on the otherhand is great. A bottle will last you over a year, and in my experience if you apply it right it lasts over a month (depending on conditions) and you’ll rarely if ever need your wipers.
In that vein, if you get the impression that it wears off too soon you’re either A) using the wipers too much and scuffing the surface back up again or B) not putting enough on or C) both.
When you apply it make sure that your windshield is clean (and if its old and beat up you may be SOL) and dry. I like to cascade the Rain-X down the top edge and let it sheet across the window and before it reaches the bottom I’ll sweep it up with a flat folded paper towel. From there I rub the shit out of it until the streaks are gone (this takes a while and is easiest to see in the sun). I’ll then repeat on the otherside of the car and occasionally redo the entire thing again. If you put it directly on the windshield (not on the rag first) I think you get better coverage and much more milage from each bottle, but it takes longer to get the streaks out.
If you put it on right I doubt you’ll be using your wipers much again.