How long would it take to walk from San Francisco to Los Angeles. Even that almost impossible to estimate and not be off be several multiples, if you're lucky!
You're not a novice programmer, you've done it before. And if you are a novice hiker, you shouldn't be doing it alone.
Similar enough that I can make good estimates. You can too, with experience.
I can tell you, if you have a relatively long project, it's worse than that: Susie (or someone else) will probably quit. Better take that into your estimate calculation.
Remember, estimates are an outer bound; if you finish early, that's ok.