One of the better calculators out there is here:
https://bitcoinwisdom.com/litecoin/calculator

3% difficulty increase and ~90 days start time (buying it now) along with the other changes (504MH, 800W, $0.1KW/h, 40LTC cost) ROIs before the end of the year. 3% might be optimistic or pessimistic. 

I'm upset my wire transfer didn't go through in time, now all sold out. With a ROI of under a month, that hurts

My wild a** guess is that the recent 2TH/s drop in LTC hash rate equates to the June shipment of 4K L3+'s. Of course they might not have been mining at all, or they might have continuously been shipping them out, or switching to another coin, etc.