It doesn't matter how long your recipe says to keep it in primary. You keep it in primary until your gravity readings remain constant over a couple of days at least or you gain enough experience to know it's done. Do some searches and read up on using your hydrometer. It's probably a good idea to read about using a primary and secondary or just using a primary fermenter only.
Halfing a vial of yeast is not a great idea. you have underpitched, but I'm sure you'll be ok. You should've pitched the whole vial if you were not going to make a starter. Do some searches and read up on pitching rates. i'll throw out the obligatory www.mrmalty.com
as well for your pitching rate education.