A lot also depends on the type of plaster you use. Drywall compound can be mixed to almost any consistency, but it's fairly soft when dry, and tends to crack if applied too thickly. Hydrocal and dental plaster both have a catalyst in them, so will set-up within a specified time. Both dry hard and their fine texture is good for casting from patterns. For scenery, I use a product called Durabond 90. It's made by Canada Gypsum, but I believe U.S. Gypsum makes a similar product. This is a patching plaster, and the "90" refers to the setting time, in minutes. Other setting times are available. This is not as fine as Hydrocal, but is a lot cheaper: around here, about $20.00 for a 35 lb. bag. This also contains a catalyst, and will set no matter how thin or thick you mix it: very watery mixtures set in the indicated time, but may take several days to fully harden. Once hardened, this stuff is very strong, and hard to sand or carve. I used if for all of my scenery shell (over aluminum or fibreglass window screen), and also for my riverbeds, (over plywood). As with applying over dried plaster, mist some water over the wood so that it doesn't remove the moisture from the wet plaster. And Gus is right: add the plaster to the water. If you mix it in a flexible container (I use a plastic litterbox), you can flex it to remove the dried residue, making clean-up a bit easier. Save those dried bits, they make great rip-rap when broken down to a scale size.
Wayne