Hi Paul.
Welcome to the green side
)
Have a read trough the tutorial section of the forum a lot of the things U will need to know are well explained there.
Often the duration of the light is less important than intensity.Many folk are unable to dim there light units so in order to reduce lighting if needed they cut the lenght of the photoperiod.
Will be good to share more information regarding your setup.Filtration+ water circulation, light unit used (led,T5,T8) and output,what kind of plants are U growing,how the tank is stocked,how long has been running,do U have algae issues.....pictures/ video are very helpful for other members in order to advise you correctly as every tank has its own uniqueness
Do not be shy to share
Regards Konsa