Ok, mine is 7 hours at 4.5wpg. I am just trying to get a general overview of what people are doing in the hobby. Some articles read 12-14 hours and some us 5 hour photo periods with good plant growth, so my question is what are we doing? I know most of you have several tanks I'm talking about your favorite tank, the one you prize and give the most attention to.
I have never seen a chart to display this but you are getting the benefit of many, many years of experienced hobbyists. I know that if it wasn't for this site I would have a really great algae farm right now.
Remeber that part of this also has to do with the plants that you are growing and their light needs. Notice that even with lower light Trenac still uses a 9 hour photo period, this is probably for a low light tank. The plants don't need as much light but they still need a good amount of time under the light to photosynthesize and grow.
I have seen some grow tanks with only 5 hours and others with much more. Plant species all have different requirements but I have never seen a suggested photo period. There are light level ratings but no duration suggestions.
I do the same as davemonkey with a split photo period. Originally I did it so I could see my tanks both morning and evening when I'm at home. It has worked very well for me for years now and I see no reason to mess with something that's working.
So my favorite tank is 29 gallons with 65w, on 5 hours, off 4, on 5. It's also in front of a south-facing window with the blinds angled so it gets bright indirect sunlight for most of the year. In winter it tends to get very little sunlight because I'm in Oregon and it's near-perpetual gray.
My 180g tank uses 0.87 watts/gallon of HO T-5 for four hours, then an additional 2.5 watts/gallon of metal halide for three hours, then back to 0.87 wpg for another four hours. Assuming I did the math right, the total photoperiod is 11 hours.
Algae isn't usually an issue but it has been for the past three or four months. I'm still toying with that one.