It seriously depends on the type of cover you have available in the tree you're in. Here in PA for example, we have mainly tall, branchless hardwoods. Hiding in one is somewhat of a challenge, especially once all leaves have fallen. This usually means being at 20ft, sometimes more depending on the terrain and direction you expect deer to come from. If you're a midwestern guy where you mainly see those knarly fencerow type trees with tons of cover, 10-15 feet is all you'll ever need to climb.