It is nearly impossible to determine ROI with GPU mining these days, a well educated guess, clever market watch and what type of mining you into can all have a factor to play.
However, if you decide to go down this road and "invest" in mining rigs, for me it works out better to keep an eye on the Compound Annual Growth Rate (CAGR)

To calculate the CAGR of your portfolio from the period from Jan 1, 2014 to Jan 1, 2017, you would divide the final value of your portfolio by the portfolio's initial value ($19,500 / $10,000 = 1.95). Next, you would raise the result to the power of 1 divided by the number of years (1/3 or 0.3333). (taken from https://www.investopedia.com/terms/c/cagr.asp)