mr. flea was very concerned about when to go in for #2. The OBs we asked said, "Things vary but expect a second labor to be half the length of the first, and if your water breaks, hie thee to the hospital ASAP." Worked for us.

Given your first labor, unless he's poorly positioned I would expect it to go fast. If it were me, I'd head for the hospital as soon as active labor started (i.e. regular, strong contractions).