Light requirements might be different for each of these plants so Google "lemon tree light requierments " for example and find out how much time do they need in the outdoor environment. And as far as how many lights you need , someone told me that 40-50watts per square foot of growing area is good so in your case with 6 bulbs on you should be fine having 2x4 feet growing area. As your plants get taller you're probably will need to ad some more lights on the sides of the plants to provide some light for the lower foliage .

Overall I think that you just have experiment and see what light duration and intensity you need because no one can honestly tell you what's proper. There is only one sun so it's easy to tell how intense it is. With growllights there are many factors, some brands are better than others, type of reflectors you have in your fixture, growing with reflective walls or tent and how close you keep the lights to the plant.