The rule of thumb is 3,000 miles or 3 months. I now have a third car that doesn't get much mileage on it and I decided to go 3,000 miles or before 6 months. So at 5 1/2 months I changed the oil with 1,700 miles on it. So, I think waiting a little longer than 3 months is OK.
On the car that gets less miles I try to drive it to work once in a while, I have heard you want to put over 20 miles on it at a time to burn off some build up. So, if you drive less drive to take a longer trip (over 20 miles) once in a while.