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!
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.
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.