According to White/Zainashef in "Yeast", "Most starters... ...reach their maimum cell density within 12 to 18 hours. Low inoculation rates and low temperatures can both extend that time out to 36 hours or more, but the bulk of growth should always be complete within 24 hours." If you keep the yeast in the starter vessel for 8-12 hours after they reach terminal gravity, it helps them to build up their glycol reserves. Then cool it down in the fridge for 24 hours, decant the "beer" in the vessel, and pitch your yeast. You can keep it in the fridge for a week or two, but after that you should make a new starter, and wake them up again.
You can pitch the starter within 12-18 hours, but you can't decant the liquid. I think you'll be fine, just chill the yeast until you are ready to brew. I've made starters a week out, to no ill effects. Just chill them in the fridge after 36-48 hours, like Pinski said. If you've gone longer than 3 days, add a little more wort, let that finish, then chill.
