The actual text states something like "fully 30% of magic weapons shed light equivalent to a torch." Since the percentage is relating to the quantity of magic weapons as a whole, I'd say it's just at creation. Otherwise the percentage of light-shedding weapons would gradually increase over time, as newer weapons would start out 30% light-shedding, and older ones would often become light-shedding after the fact as well. Consider the light-shedding property as an occasional side-effect of that first +1 enhancement.