Sorry I missed this before. I tried to set it to send me an email with new posts, but not working.
If the ARR is expressed as a rate, then the NNT will still be its reciprocal, and it will include the duration of treatment needed. For example, if the ARR is 5% over a 3 year study then it's 5%/3 yr and when you turn it upside down it's 20x 3 years; i.e., treat 20 people for 3 years to prevent 1 event.
Does that help?