If you count the time between learning that there was such a thing as a Walther P.38 in .22LR, and actually rounding up all the parts and building one, right around 40 years (1966-2006).
The time between deciding I wanted a vintage Rem 700 Varmint in either .308 or .243 and putting away the money for it, and finding a suitable candidate priced right (a very clean 1969 build)--somewhere around 3 or 4 years.
Time I spent lusting after a Springfield Armory M1A, only to find that every time I saved up the price, they'd raised it another 20-25%, until I finally bought a PolyTech M14S--about 20 years (@1973-1993).