Regularly.
With my mom and sisters, leaving the kids with my husband - because Daddy time is important for the kids and time with my mom and sisters is important to me.
With my girlfriends, leaving the kids with my husbands - because my girlfriends are precious to me (and the kids get more Daddy time).
But especially with my husband, because after the kids are gone I want to continue to be married to someone I enjoy and that I have a lot in common with. Marriage takes work, and spending a long weekend (or even a week) together on vacation is (IMO) a great way to keep a marriage strong and healthy. Its easy to drift apart during the 20 years you spend raising children - I don't want to look across the table from him in 12 years and think "why am I married to you?" Perhaps not everyone needs a few "couple days" to keep their marriage healthy, but it seems to help. Besides, the kids get a vacation at Grandma's house - which they beg for.