Welcome to the forum!
This is service used by real people, so it is impossible to build an universal formula. Nobody can guarantee that all provided space will be filled fast.
We always suggesting to do not invest to anything with purpose only for Storj, you could not have a ROI any time soon. It’s profitable from day zero if your hardware is already online and all costs paid anyway. In such a case any income is a pure profit.
I would like to suggest you to use this Community Estimator to have an idea:
Copy the spreadsheet to your account and place your numbers. Take a look on needed time to fill up the space and note how much you can earn with the current behavior of the network.