So, that averages ~6 months to break-even on an average employee, 9-12 months if you include "duds" in the average.
IMO your latter example is something very wrong - either on the employee's side (they are lazy and not doing anything) or management who hired too many employees and as a result those new people have nothing to do - either way, heads should be rolling somewhere.
let's say you hired two researchers to do some machine learning experiments to develop a new product category. It could easily be years before you see any ROI from that. Especially if their work has to be integrated with the rest of the (functioning) system.
or say you hired someone for their proficiency in one part of the system -- say, they've run a top level domain before, know DNSSEC/IANA/ICANN in and out, etc. If they never get to work on that, or their code never goes to production because of other complications, it doesn't ding ROI. And yet, they are technically doing what they were hired to do..
i guess my point is that ROI is the final output of many, many different factors that determine an employees usefulness and relevance to an organization.