If I buy a vehicle that's had regular fluid in it, or using a work truck where someone used standard fluid on the last fill, I'll let it get low and then do a complete fill of the Rain-X. First time it's raining or I need the washers, I'll run the washers for several cycles and I typically start to see results right away.
I'll note that while it does let the water bead up, it's not very useful at low speeds. However it is very handy at 35+ speeds when the water just beads up and blows off.
Oh, I've also found that the Squarebody trucks don't have proper aerodynamics to fully take advantage of the beading/blowing action, lol.