Well, one of the requirements for you to be able to claim someone as a dependent is that he/she must be a U.S. citizen or resident alien. So the key issue here is whether your parents are resident aliens.
Someone is a resident alien if they pass either the substantial presence test, or the green card test. Your parents don't have green cards, so it's the substantial presence test that is relevant. This is where how long they've stayed in the U.S. is important.
They satisfy the substantial presence test for 2013 if: (number of days they were present in the U.S. in 2013) + 1/3 of (number of days they were present in the U.S. in 2012) + 1/6 of (number of days they were present in the U.S. in 2011) >= 183 days.
Do they satisfy that? If they do, then they are resident aliens for 2013. If they are resident aliens, then they can be claimed as dependents.
Note-1: if they are resident aliens, all their worldwide income would be subject to U.S. taxes.
Note-2: As per this IRS link,to pass substantial presence test, you have to be in USA atleast 30 days for the current year in addition 183 days calculation which includes current year and last two years/.