I formerly owned a black Mitsubishi Eclipse before I purchased my white RX 300. When I sold my Eclipse, the paint was still as sharp as the day I purchased it. One thing that would help is to try to keep it out of the sun. Depending on where you live, the elements can really cause a lot of paint fade. I use car covers on my cars when I need to park them outside. They're a pain in the butt, but they really help keep the cars looking good.
Like everyone else, I would also keep up on your polishing and waxing. I usually use Mother's or McGuire's wax, but my only authority is what others have told me. Additionally, you may want to use a sun screen to help keep the dash / car cooler. For giggles I put a thermometer in my car to see if it made a difference, and it was about 30 degrees cooler (120 degrees).
As for time dedicated to cleaning a car, I don't think there is an excessive threshold. My wife like to spend hours watching re-runs of Friends and Buffy ... so what's the problem with spending some time on your car? Every year I spend about two days cleaning my cars including washing, waxing, polishing, and detailing the engine. If possible I even remove the seats and carpet clean the entire vehicle. This may be excessive to some, but I enjoy it. I just put on some good music and go to it. My uncle is a doctor with several successful offices around the Dallas area, and he still changes his own oil and details his own cars. If he can do it, I certainly can find the time. :)
Congrats on your car! Lexus makes an awesome product and I'm always proud to be in mine.
D.J.