Oil change frequency

Hello,

I own a 2001 honda civic and 2006 odyssey. My question is how often do you need to change the oil for these vehicles? Should I just go by the mileage alone or does the number of months matter? For the latter, I know the recommended # of months is 3, but I rarely ever getting any where even close to the 3000 miles mark. So what is your suggestions?

Thanks!