The actual time taken depends on various factors such as the weather, air traffic delays etc. However, airlines normally allow about an extra half hour for westbound flights as you are flying into the prevailing westerly winds.
usually it can be a little faster coming back as it is fron the states,as gef says a number of factors make it that way,but i expect mr know it all will dispute this later
To settle any arguments, there's a truly excellent site here.
It even works out connecting flights to smaller airports, so long as such flights are available!