6 days was my longest, which includes the time to let it settle out so I could decant it. That was a 3L starter for a lager, which take longer in general because of the lager yeast and the size of the starter.
Yesterday I pitched a 4 day old starter of American Wheat. This was again 3L, going towards an 11 gallon batch. The smack pack was about 9 weeks old (~50% dead). It took it awhile to get started but eventually foamed over and made a mess of my stir plate. Pitched yesterday and today it is fermenting strongly with a beautiful white krausen.