Tom,
We bought the property in 1996. We have just over 10 acres, with about 5 of them covered with what were x-mas trees. We are the 3rd owners. The first owner planted most of the trees several years earlier. My guess is that they were initially planted 7-9 years before we bought it. The trees are a mix of Norway Spruce, Blue Spruce and Scotch Pine, with a handful of Douglas and Fraser Firs scattered throughout. These Firs have been "spot" planted in sparse/harvested areas by the second owner. Now I have many large trees that are not suitable for x-mas, and cannot be used for state landscape projects (due to trimming), and are cost prohibitive to use in quantity for landscape resale. I have begun cutting them down and burning them/w3tcompact/icons/sad.gif. If I were just starting out (or starting over), these are the main points I would consider:
Choose trees that sell well in the local nurseries/Lowes at Christmas. Preference in tree species is VERY regional, and also tends to shift over time. Here, the Doug/Fraser Firs sell best, since they have soft needles. Unfortunately, deer love these best selling trees because they have soft needles. It is not unusual to loose 95-99% of first year plantings to deer. They prefer young fir trees over just about everything else, year round.
Choose trees that are more "deer hardy". Spruces are also a popular seller since they have a great smell and retain needles well. Deer don't like to eat them because the needles are very sharp. They also have a natural pyramid shape, and sell well even if not trimmed (i.e. low maintenance)
Plant 2 species to meet more preferences.
I would not recommend investing the energy on anything less than 3 year seedlings, unless you have the experience and conditions of a real tree nursery. Do not plant different age seedlings in order to stagger your harvest. It is better to stagger planting areas each year. Also once the seedlings are planted, they grow at different rates. Some will grow rapidly and be harvestable in 4 years, while others won't even reach 5 feet in 8 years. Yes, the majority of them will mature within 2 years of each other, but like people, they all grow somewhat differently.
Better yet, plant 1 or 1.5 acres each year. By rotating these lots, you can stagger the harvest more effectively.
If you are planning on trimming them yourself, and you have a day job, I would recommend that you keep your crop relatively small, as it is VERY hard work to trim more than a few trees at a time.
For a hobby farmer, I would recommend half-acre lots staggered over 4 years, for a total of 2 acres of trees. This is a size that would be comfortable for me, since I really only set out the cut your own sign on weekends during Advent. Any more trees than that, and I would have the same problem I have now - a growing forest that threatens to block my view of the valley below.
I plan to expand my blueberries and add grapes. That way I have some cash crops that have different seasons of work effort. By far, the hardest thing in front of me is clearing the land of the overgrown trees to make way for vines and berries.