Clients will ALWAYS notice when the band takes a break, but never notice how long a playing set might be ...
My nephew recently got married, and admittedly there was a lack of communication between him and the band early on that caused some problems, and he was now kind of 'on edge'. The night of the wedding after a long dance set the band took a break ... they were 20 minutes into it when he said to me "all this band is doing is taking breaks" ... he was kind of surprised when I mentioned that they had played for almost 1 and 1/2 hours straight !!! ...
However, I have seen bands take advantage with long breaks, but then I've seen DJs spin nothing but 'dinner music' the first 2 hours also....

To Tony R: ... like anything else, the $$$ a band gets depends on a lot of factors ... In the greater NYC area top wedding bands (and I'm talking about professional musicians/singers who can and will play ANYTHING you ask for) demand 5 to 8 or 9 THOUSAND a night !!!
t.