How long it takes depends on how you build it. With a model-only level, even extremely large and complex levels take but one second to build. If you used snap, you can save more build time. A level with 3000 blocks with snap used would take anywhere from 3 minutes to even a week, depending on the build options. If you didn't use snap, the time could be doubled. I suggest using models for everything you can as, not only do they render extremely fast, but models also give much more flexibility in shape and size, along with far shorter build times. Sure, you can use blocks, especially for walls that block the view of other things, but with yet another trick, though it takes more programming to do so, you can hide invisible objects when not visible.