Answer: According to Dirr's Manual of Woody Landscape Plants, the Leyland Cypress, while being a fast grower, is not long-lived. They suggest 10 to 20 years as a possible lifespan. It seems this is often the case with quick growers.
Depending on what you're looking for in your screen (height, density, etc.) you might consider eastern red cedar, (Juniperus virginiana; medium growth rate, height 40-50', spread 8-20'); Japanese cedar (Cryptomeria japonica; medium growth rate, height 50-60', spread 20-30') or one of the various yews or spruce. Spacing will depend on the plant you choose, and how dense a screen you want. One option is to plant your row of permanent trees as you like, then plant some faster growing, but shorter-lived, trees in a row in front of those. Poplars, for example, can grow 4-5' a year.
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