Maybe they lied, or you are running intense apps, go to apps and see what's running, you don't say how quick is quick, During the day, I set phone Airplane mode until break time.
It really depends on battery use, as Golfdriver said. Post both screens of battery use. (Press the graph at the top of the first one to get the second one.) If you need more use out of the battery than it can supply (constant use of a phone won't give you anywhere near 8 hours) you can get large external batteries that plug into the microUSB port, or you can buy a spare battery or two, with a charger than can charge both the phone and a battery not in the phone, and switch batteries during the day.
(BTW, if you're constantly draining the battery all the way down, you're killing it. As a general rule [you don't have to be obsessive about it], the phone should be recharged when it gets down to 40%-60% of charge. Discharging it much less than that regularly shortens its life. See Battery University - How to Prolong Lithium-based Batteries)