We need to know what plants you have, how many gallons of water you have and the watt output of that light
Exactly! Some people go by the whole watts per gallon rule but that to me is very misleading. Just like when dealing with an indoor garden, you deal not with wattage per ft2 but rather LUMENS per ft2. Same thing I would think applies to a planted tank. Rather than going by how many watts per gallon, you should be more worried about lumens per gallon. A lumen is how bright the light is, and depending on the type of lighting, you have to consider the different lumens per watt output of that particular light. For example, you may have 3 watts per gallon of incandescent, but 3 watts per gallon of flourescent will be much brighter cause a flourescent has a much higher lumen/watt ratio than an incandescent.
Now it gets more complicated if you think about this for a second. Cause what determines the lumens per gallon? Well just like Crepe said, that is determined by the type or types of plants you are growing. Some require bright light some dont. If you have a mix then I would consider maybe an average of the total lumens needed for your mix of plants. Im sure if you dug deep enough on google you will probably find growth parameters of your particular plants that list an ideal lumen ratio.
ccfls are the way to go. They require only a fraction of the wattage of a standard flourescent and they are very bright, although not as bright or as effecient as a regular flourescent but they are very compact and do very well for their size. Also they are "instant on" lights, they dont require a warm up time for cathodes to heat like in regular flourescents.
My ccfl I [moderator edit]consumes about 8 watts (total, as in the whole circuit takes about 8 watts but the actuall bulb consumes less), and small ccfls like the one I use put out around 25-30 lumens per watt. So lets guestimate and say its about 27 lumens per watt. At 8 watts your looking at about 216 lumens of light. For my small as 10g, thats 21.6 lumens per gallon with just the ccfl. Your average tungsten filament 120V incandescent will probably put out about 13 lumens per watt at best. So 30 watts of incandescents that I have along with the ccfl provides only about 390 lumens total (but with 22 extra watts being wasted), which is about 39 lumens per gallon for my 10g. If you add the two together, you get a total of ~38 watts of power consumption, with a total of about 606 lumens.
So with my setup I have about 60.6 lumens of mixed lighting per gallon, and I would say its about very slightly above average. I just have apono's and a lily growing and they have practically taken over my tank with just 60.6 lumens per gal
But the types of plants matter greatly, cause obviously some types will absorb more light than others. It gets more complicated cause you also have to think about the spectrum of light each plant requires. As a general rule of thumb I learned from indoor plant growing is you wanna stay away from the green colors and stick more with then reds and blues of the spectrum.
The easiest way to tell roughly the spectrum required is to simply look at the plants you are growing. What color are they? A green plant looks green cause it absorbs all other colors of light but reflects the green spectrum. This green light is reflected into your eyes and hence the plant looks green. So if you have say a red lily and look at its leaves, you'll notice they are reddish green in color. That means that particular lily reflects more of the red and green colors of the spectrum so obviously lights in this range will be of less use than a light higher in the blue side of the spectrum. If you have a mix then its best to provide a good spectrum "bandwidth" of mainly reds and blues.
Now as far as the amount of time the light stays on, well that all depends on a number of factors also. Your best bet is to add you some fish to that tank, mainly something like a catfish or a pleco. When I first let my tank cycle I didnt clean it for like 2 months and I only had the light on no more than 12 hours a day and it was overrun with algae!
However, since I bought my pleco that tank has been spotless! He will scour every inch of that tank till there is no more algae left! So for crap on the bottom I let my catfish take care of that (to an extent), and algae is taken care of by my pleco.
I routinely leave my lights on for 15 hours or so (I know Im trying not to make a habit of it cause I know it stresses the fish more). I have been doing this for like two weeks and my plants have exploded in growth! No worry about algae as my pleco takes care of that. I will turn them on when I get up about 12:30-1pm, then I will go to sleep about 4am or so and I just leave em on till I wake up to use the bathroom in the 6-8am range, then I turn em off and go back to sleep. So thats what...[moderator edit] like 18 hours! Yeah I got to stop that cause I dont want my fish getting stressed but the plants love it!
So I would say probably stick to the 12-15 hour range. This also depends on the plants you are growing. I have no idea if some of the aquatic plants are photoperiod plants but I imagine some probably are so you have to watch out for that as different amounts of light cause photoperiod plants to change their growth cycle (veg to reproduction [flowering] and vice versa).