From what I have learned, becoming implies a permanent change. So the weather will permanently become cavok. The time stated is the time frame at which the weather will become cavok, so in this case, the weather may become cavok at 12pm, 3pm or 9pm.
Remember tafs are forecasts and not actual meteorological measurements at the time of the taf, so it's just an educated guess.
Hope that helps, hope I'm correct.