For countries it takes 1 game month for a corporation to be built. Only 1 gets built per game month unless you buy the booster.
For CEO, if you build in a computer controlled country it takes 1 game month and again only 1 built at a time. If you build in a player owned country, you have to wait for them to approve before it builds. If they are inactive, I believe it auto approves after 12 game months (2 real days). And again only 1 can be built in a game month unless you have a booster.
One other possible explanation is that you do not have any production plants in inventory and there is a shortage on the market. This has never really been an issue for me, but you can double check your stock of production plants on trade screen.
Glory to the Pinktator!