Never kept track of exactly when cabin lights are dimmed, but this is a daytime flight, so the cabin lights won't be all that bright at any time. The sun, however, will be at full tilt the whole way. Typically, the FAs do the "let's close the window shades and crank up the temp so everyone goes to sleep" routine shortly after lunch is served. IME, this happens as soon as the lunch service is finished, about one and half to two hours after takeoff.