The ROI for the Antminer L3+, 504MH/s is around a month right now with current difficulty / USD/LTC exchange rate with free power
BUT keep that in your mind , if you are ordering those miners right now , you going to get them around 20-30 of July . That is 2 and a half months away , difficulty by then probably going to be much more higher. So 1 months ROI only for those guys who ordered from the first or second batch :-) and already mining with those miners
I have a few , each can mine 1.5 - 1.6 LTC / 24hours
My guess is , by the end of July those miners ROI will be around 4-6 months , time will tell