It seemed like my barrel got little or no takers for weeks, if not months. But once the heat of summer rolled in, they started wearing it out. I know the bees are hard to figure out at times, but I just can't imagine them opting for a river 1/2 mile away if you give them a viable option within 10 to 15 feet.
The half barrel really works well, I can leave it for extend periods without worrying about the water level. One of the most important things concerning a water source for bees it that it be dependable and constantly available. It's my understanding that they will sacrifice convenience for reliability.