Gmail only polls each POP account every now and then. Sometimes, yes, it can be several hours between polling. Other times it seems to wait only a few minutes. At least on their regular HTTP site, you can manually force a POP check to any given account at any point, like if you know you have an email coming.
On the Blackberry application, I'm not sure if there is a way to force a check like that. You kind of just have to wait around, and when Gmail checks your account, it will download the message.