The average your oil should be changed is every 3, 000 miles.  However, there are signs your oil should be changed, even if it is before that 3, 000 mile baseline.

The oil looks black and gritty.

Fresh, clean oil is the color of a honey-brown that will darken only after a few weeks of use.  If upon inspection you see particles mixed in with the black oil, you need to change your oil.

Your engine seems to be louder than usual.

Oil lubricates your engine.  If you require an oil change, meaning you’ve lost that lubrication, the guts of your engine will start to grind against each other.  This could lead to other expensive problems.  Change your oil asap.

It’s been so long that you can’t even remember when your last oil change was.

This sign is self-explanatory.  Change it asap and write the date down.  It is best to stick to a regular maintenance schedule.

Your check engine lights won’t turn off.

The check engine light could mean more than one thing.  However, it is best to check your oil if this happens to ensure you are lubricating your engine appropriately.

The oil level has dropped.

Over time, the engine will “eat up” the oil, decreasing the level.  When this happens, it won’t be doing its job to the best of its ability.  Upon this discovery, don’t just fill up on oil.  Check the oil for any grittiness.  You may need to change your oil.

Source:
Markovich, Tony (march 15, 2013).  5 Signs You Need To Change Your Oil.  Retrieved from http://www.complexmag.ca/rides/2013/03/5-signs-you-need-to-change-your-oil/