That 5 min or better thing is a blasphemy that some people came up with and are repeating for the reason I don’t know. For maximum profits you should have frequency as bad as possible without breaking the 20 minute rule. Longer frequency means the factory will be able to produce more in the time the train isn’t at the station and when the train will arrive it will have more to transport and thus you will be able to run longer trains. Fewer and longer trains are more efficient because wagons have much lower running costs than locomotives.
So with very long distances the frequency will indeed have to be at something close to 5 minutes because if it takes a train over 10 minutes to travel to its final destination then there will be little time left for loading and unloading but on shorter routes frequency can be as bad as 10 or more minutes.
Take a look at this 10min frequency iron line:
