It's hard to give a precise answer , but for something like that on the more elaborate end of what I do it's probably an evening.

There are a few tricks to cut down on the time by a great deal though! Big blocks of foliage are a basically a cheat code to fill in areas where I don't know what I'm doing. Also, working in simple blocks of color with gradients to fill in further away areas helps create depth and fills in space real fast. No joke, watching Bob Ross episodes are a massive help too.