My key point every time my team discusses hiring: If you think your take-home project takes N hours, add 1-1.5. You already know the context, and are underestimating the difficulty even if you've run through it before.
We're doing a hackathon-style interview session this Friday, and I've already had to remind the team that one of our coding challenges is "easy" because we know the answer. You need to think back to the first time you saw it. Also, we're not actually interested in the solution, but how you arrived at it. Once you grok the problem, most software people hit on the solution pretty soon, and can begin coding - we want to see how you come to the realization, and then how you work to create the solution.