When they were little (starting when my oldest was a baby), we'd get one getaway a year without kids. Sometimes we left them all (loved those trips), sometimes just the diapered set. As a SAHM to 5 kids very close in age, I needed to get away with just my DH, and breath (and sleep). Now that they are older, and funds are limited, we just do family vacations, because I enjoy them so much now (ages 14 - 21), and time together is fleeting. I think the last time we went away alone was probably 8 or so years ago.
When they were little, we had baby sitters all of the time (ok, like once a week), either grandparents or teens. The older they got, the less we left them, although now it's rare for them to be home on the weekends (and weeknight due to activities, plus the oldest are away at college).