You do it when no when is looking (hence robber baron).
Best to start when the land is cheap, and when you are going to build infrastructure to increase the value of the property. So the railroads with tracks and stations, amusement park operators (Disney World...), or it could be a factory with lots of jobs in a depressed area.
You said they would "buy up all the property in a constrained space to corner the market". I don't see how your examples apply. What constrained space did Disney World take over? What market did they corner?
Best to start when the land is cheap, and when you are going to build infrastructure to increase the value of the property. So the railroads with tracks and stations, amusement park operators (Disney World...), or it could be a factory with lots of jobs in a depressed area.