I'd wait minimum of two weeks. Three would be better, four would be great. Porters need time to age a little before consumption, and batch aging is the best way to go.
Personally, I usually don't even bother taking a gravity reading until it's been in the fermenter for two weeks, sometimes even three.
theres no need for a secondary unless you plan on leaving it in there for months, just leave it in the primary for at least 3-4 total weeks and move on to bottling