One thing I've noticed on this forum since getting into mining a number of months ago is the pervasive use of ROI (Return on Investment) to incorrectly refer to how much time is needed for mining profit (revenue less expenses) to exceed the original cost of the miner, power supply, and other related start up costs.
People can use words how they like, but in the rest of the financial world ROI is generally defined as annual profit divided by the investment incurred to produce the profit—expressed as a percentage. For example if I put $1,000 in a savings account at the beginning of the year and I receive a total of $20 in interest by the end of the year, my ROI would be 2% (20/1000).
What people mean most of the time when they use ROI on this board is "Payback Period". For example, say I bought a used S3 and PS at the beginning of the year for $150 and it took me six months to earn $150 profit from this S3; I would have a 6 month Payback Period for this S3. If I made $300 in profit for the entire year, I would have a 200% ROI on this S3 (300/150).
I'm not trying to be the acronym police, but I wanted to let you know why other people that are not on this forum might look at you funny when you talk about a 3 month ROI.
Here is one of many sites that will explain the difference in financial performance measures like ROI and Payback Period.
https://www.business-case-analysis.com/return-on-investment.html#compareBTW: if you are considering making a large investment in a farm vs. some other investment, you should get to know what NPV means and how to calculate it.