Tyson,
The trees I build for the modules take from 1 to 4 hours each to build(one of the pines took 8 hours to complete). The forest on the modules has grown steadily over a fifteen year period. Because I have only 24" depth of scene, on public display, I chose to "model the forest". For a layout, especially for background, I'd use the same technique you've used, and save myself tons of time.
Pete