I think it varies depending on the species of flower. Some flowers are evolved to be pollinated by bees, or hummingbirds, moths, or bats, or whatever. This effects how much nectar they secrete, its composition and sugar concentration, and timing. I found an interesting article. I didn't get the chance to read all of it, but it talks quite a bit about the various patterns of nectar secretion. If you can wade through it, there's quite a bit of information.
I thought it was particularly interesting that certain flowers will produce nectar up to a certain point, and then when the nectar is removed, they respond by producing more. I don't know how they "know" the nectar has been drunk, but somehow they do. Hummingbird pollinated flowers tend to time the beginning of their nectar production so that there will be plenty of nectar at first light, when the hummingbirds first arrive. At night, they may not produce any nectar at all. Bat pollinated flowers, or ones pollinated by night flying moths may not produce until late in the day and into the evening. It looks like nature has devised all kinds of wonderful strategies for amounts of nectar, composition of nectar, sweetness, timing of production, and whether or not they produce more nectar when they have been depleted, or whether they just go dry and stay that way.
When I get a chance, I'm going to try to finish this article. It starts on p. 80, and I think I got to about page 102.
I'm not sure if this link is going to work. At first it did, then I edited something, came back, entered it again, and now I can't make it work.
http://books.google.com/books?hl=en&lr=&id=9ULQh89IINgC&oi=fnd&pg=PA80&dq=nectar+production+flowers&ots=qNsPP6hX-g&sig=pSySAlVGYu6UnHcyCYlsJUPqcBo