How much power do LED smart bulb wireless control systems consume when the light is turned off?
The specs are out there, just not always so easy to find. For example, one Philips Hue is rated at 9.5W with max. standby of 0.2W. (Click "The bulb" on the right side under Technical Specifications).

Using this bulb as an example, if it actually uses the full 0.2W all the time, 18H x 365D x 0.2W = 1,314 Wh per year. If your electricity costs 0.10/kWh, that's 13 cents per year for the 18 hours of standby vs. $2.08 for the 6 hours of light.