This is only a rule of thumb but RH for veg is considered to be around 60%RH in flower this should drop to around 40%RH.
Personally I don't worry about this but I alway keep an eye on RH, it never goes above 60%RH
Ways to reduce RH are to use a dehumidifier or increase extraction with a larger fan and also make sure that the negative pressure inside the cab isn't too high.
If the negative pressure is too high it could be because the intake is not supplying enough fresh air and the extractor is trying to pull new air in against this pressure and it will put strain on your extractor fan.
some fan speed controllers will have humidity assist which will ramp up fan speed by 20% to help lower RH and then slow down again once the ideal RH is reached.
An example of this is the Progrow Multi-Fan Controller, which is on my Christmas list!
I also run 24/7 as Fairly new mentioned this keeps a stable environment, which is useful for a couple of reasons, when a plant is in stretch mode the higher the difference in lights on temps and lights off temps the more stretch you will get.
High humidity combined with high temps can also cause etiolation, which is stretching of the stem, similar to the stretch plants get when not receiving enough light.
A cold wall when lights go off can lead to condensation forming on the walls as the moisture in the air forms drops of water on the surface of the wall, which can lead to powdery mildew, botrytis etc . some fan speed controllers have a setting for lights off and will continue to run the fans on a higher speed for an hour, so to avoid these condensation problem.
