Lighting is considered a continuous load.
210.19(A) and 210.20(A) limit continuous loads to 80% circuit capacity, so...
A 120-volt, 20-amp lighting circuit is limited to 1920 watts.
A 277-volt, 20-amp lighting circuit is limited to 4432 watts.
If you are using fluorescent or HID lighting don't forget to add ballast load.