Working on it myself, after having read what the code does and being committed to re-implementing it myself, it took me 3.5 minutes and there were 2 bugs. (I needed to look at the original code again to determine that to == length and to == from were both allowed -- those were the bugs.)
Did you include the time it took to read what was needed, including the different exceptions? I would estimate, if one wasn't fresh and up for a challenge, 15 minutes (against your approx 10 minutes). And given that a day of programming isn't pure programming, I'd double it to 30 minutes. It feels way too long, but it's a realistic, day-in-day-out estimation.
Programming takes time... which is why it's so powerful if you can reuse code (as Bloch did), and even better if you can find a way to reduce the code needed, or best, to avoid needing any code.
A set of tests took me about 6 minutes.