Answer:
Naturally: only during daylight hours
Artificially: all 24 hours (under perfect circumstances)
Explanation:
Photosynthesis can only occur when a plant receives energy from a light source. Naturally, the light source would be the sun which is only around for a certain amount of hours. If the plant is under an artificial light source, like a lamp, then photosynthesis can occur at all hours of the day.
