Effect of Low credit hours/ long undergrad on grad school
I don't think taking a little bit of extra time is going to make a huge difference in your case is viewed by admissions staff, so long as you've been putting the extra time to good use. Have you used the extra time to do research? Have you done summer internships or research programs? 

If all you've done is lengthen your undergraduate program with nothing but good coursework to show for it, then extra time may hurt you.