You'll get conflicting opinions I think. Many people will say Lexus just did a refresh with the spindle grill and LED DRL's in 2013 so they probably won't redesign it until 2016 model. I, on the other hand, think it's time for the next generation; although, I don't know how much of a redesign there will be. One thing to watch out for is the new Remote Touch Touchpad Controller, which I would love to see how it works compared to the mouse thing there is right now. Lexus is at 2 car shows right now (one in Japan and one in China) and we should know more about the new remote touch by the end of the weekend (I hope anyway). There is a teaser video for the new remote touch (but it doesn't really show how it works). http://paultan.org/2013/11/20/lexus-remote-touch-video/
IS on an 8 year run. The prior ES was on a 6 year run. Gen 2 RX was on a 6 year model run and I expect the same with the current Gen 3.
Also, with the new NX around the corner to be on the market in a year from now as a 2015, sized just a few inches shorter than the RX, I expect the next RX to be unveiled just 6-9 months after in the following model year by the summer 2015 as a 2016 model, but no later as the NX will compete too closely with the current RX.