20 years of storage
I can just beat 21 years.
My wife gave me Guillow's P51 for Chirstmas two years ago. It got me excited about building again. Last Easter, I pulled a half finished damaged Jetco Thermic 36 glider from the rafter's of my Dad's workshop (It was Christmas gift from my brother). I was 12 when last looked at it.
Using the plans and scaled the unbroken ribs as profile templates, I scratch built a second glider. I flew it with my son at the local Junior High last summer. I'm 34 now.
22 years.
I'm building a GP Spirit right now and have other kits waiting for the building board.