Dark oil shows that the detergents in the oil are working well--it's not necessarily a sign that the oil needs to be changed. If you really want a better idea of when you should change your oil, have it analyzed by a lab when you change it to see how well the additive package is holding up between changes. Blackstone labs will send you an analysis kit for free if you request one online.
At the very least
, follow the recommendation in your manual (below) and change it when the OLM comes on, at 10,000 miles or 1 year, whichever comes first.