Thanks chriswhy1967.
I know you directed the question at Sue but I'll offer my opinions.
The length of time to put your lighting on for will depend on many things.
How bright are your lights?
What kind of plants are you growing?
Are you looking to maximise growth or simply maintain healthy plants?
Are you seeing any algae?
Are you fertilising?
To make your decision you should have a good look at your plants and decide of you think they're growing well and looking healthy. Do you have to trim them to keep them under control?
If the answer is no then you can make a change. Perhaps increasing lighting will help - try adding an extra hour a day.
If that doesn't show any improvement try something else.
Maybe you need to add more (some) fertiliser...
Perhaps there's not enough carbon for the plants needs - here you can add a liquid Carbon product like EasyCarbo or Flourish Excel, or you can look at a CO² injection system.
If something works, stick to it.
If you think things could be better, make a change.
If you do make changes, do so gradually and watch the results.
I'd never advise someone that's currently putting their lights on for 6 hours a day to instantly change this to 12 hours. Try 7 hours a day and watch your plants to see if things are improving, staying the same or if their health is declining.
If you make a change decide after a week if you see any improvement.
Don't change one thing on Monday and then something else on Wednesday because that way you won't know which change has had an effect.
I'm not an expert, I've done a lot of reading and through continual evaluation and trial and error, I've learned what's working in my own tank, but every aquarium is different with varying water quality, different plants, different lights etc.
So every aquarium will have different needs.
I do think that natural plants add so much to the aquarium, not only in terms of looks but also in adding huge benefits to the fish and the ecosystem you're providing them.