As a working professional my time is already stretched thin as it is so often times the longer the better.
48-72 hour jams are fun, but they're too big a commitment for me most of the time and require well-in-advanced planning even though it's my preferred length. Typically though, the sweet spot is around 14 days (2 weeks) for me, as it allows me time to get in some coding through the evenings in the week, and a bit throughout the weekend.