I’ve been working on the same medium-size (fewer than 1M LoC) codebase for about 7 years now. I feel like over the years, my estimates of how long something will take have gotten better for one reason: I’ve found the scaling factor I have to apply to my intuitive estimate that brings into the realm of reason.

So, if I think something looks like about a day’s work, I’ll actually estimate it at about 3.5 or 4 days. Thus, for a project to qualify as “just an afternoon,” I’d have to naively estimate it at under an hour.

I rarely have time to spare, but I also rarely go over by more than maybe a third.

Your multiplier may vary depending on how horrifying your codebase is. On a side project with good test coverage, my multiplier is only about 2.