Because of the complexity and the importance of getting it right, it took her around 10-15 minutes every time she needed to use it. She, and her colleagues, are pretty much luddites so a basic calculator was their only tool.
I spent 10 minutes writing the formula in javascript, dropping it an html file, and running a few test.
She absolutely loved it, and shared it with the rest of the staff. Apparently some counsellors at another clinic heard about it and requested it as well. Someone thanked me at her Christmas staff party.
In terms of effort to build vs. hours saved and end user appreciation, I don't think I'll ever top it.
About the last point you made, how it would be great if hardware/mechanical devices could be made more like software - a few years ago I heard about "micro factories" (forgot the exact term): 3D printers for small(er) scale manufacturing. It seemed promising that soon we could be writing software to manufacture devices/products at home, in a garage, local maker "lab". If you thought of an easier mechanism to mince garlic, you could whip up a prototype in a code editor and "print out" a working prototype (or production-level device)..
I think one of the most valuable thing software engineers can do for themselves in some circumstances is attempt to shift perspective towards assessing projects by how many people they help and how. The most rewarding projects I can think of are the ones where a day or two after rolling them out, I'm contacted by a few employees that they affected the most and told they love it and it saves them 30-90 minutes every day, consistently.
I think it's a feeling most developers can empathize with, since we often automate boring drudge work in our own jobs and lives. Making multiple people's lives and jobs a little less monotonous and dreary, or a little more effective at their job in a way they feel and appreciate is a good feeling.