Oh, that's an interesting question! The answer depends on several factors. If you're planting a seedling or sapling, it could take anywhere from 5 to 15 years to reach a height of 6 feet. However, if you're planting a mature Christmas tree, it could take up to 20 years to reach its full size. So, it's best to plan ahead and give your Christmas tree enough time to reach the right height for your decorating needs.