Okay, since I have enough of this no ROI crap, here is a little hint:
even at 700.000.000 difficulty you have an ROI. Granted, if you buy an asic at that point it takes 1 year and a half, but ROI is still possible.
If the cost of electricity(*) exceeds the value of the mined BTCs you´ll never have ROI.
(*) There is no such thing as free lunch/electricity
smartass. the calculation was made with 0,31 usd/kwh. That is German standard and most countries will be lower than that.
And, just for the fun of it: with 0,15 usd/kwh which would be an US standard you get your ROI after 1 year.
The difficulty rises constantly while mining ... after X days mining won´t be profitable anymore. If X < 1 1/2 years you dont have ROI.
Let´s get back to the topic.