Around 12 hours is recommended - might save you a bit of money on your electricity bill!
Does you Beardie stay awake until 10pm? Ozzy doesn't! He gets himself settles by about 7pm and pretty much waits for the lights to go off.

I would have them go off around 8pm, but earlier in the winter, as the night are naturally shorter, you should recreate this, so maybe switch off at 6pm?