This command toggles instant construction for your nation (and other human nations) only - AI nations do not have instant construction. Instant construction makes building finish in one day (almost instantly).
The syntax for the inc command is as follows:
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Find below working examples of the inc command.
Note that this command is not the same as the instantconstruction command. If you have instant construction off (default), typing the above command into the console would enable instant construction for yourself and other non-AI players of the game. If you have turned instant construction on already, typing the above command would toggle instant construction off. With instant construction on, buildings finish in one day, bypassing their usual build time.