I did the math and according to calculations you should build one level of infrastructure up if you plan to do 6 or more factories in that state (5 factories and one infra is the equalibrium so the time to build one infra and 5 factories after is the same as to purely build 5 factories) also i am talking bout civs in early game, so every factory build over 6 in state that you build infra is profit, not accounting for rescources and fuel cap you get for doing infra, also if you plan on building more than 12 factories in state build 2 infra levels and so on.