I was transferred to Non-Active Reserve status on 5 DEC 91, and enlisted in the Air National Guard on 5 APR 2003. I'm sure that some of you have had, or have known someone with, longer breaks than that.
I work for the VA in disability and I saw a Army infantry Vet who served in Vietnam that was discharged in 1969 join the Army guard in 2003 and deploy to Afghanistan as a doctor. I thought that was pretty cool.
Fun fact...many senior officers actually retired, and then were called back in. For instance, MacArthur retired as Chief of Staff of the Army in 1937. He was called back in 1941 and fought WWII and Korea. He retired again after 50+ years of service.