The BackBerry 8700 uses Lithium Ion batteries. They do not build a "memory" like the nickel-cadnium batteries do, nor do they need discharging like the NiCads. In fact, a deep discharge is harmful to Lithium Ion batteries - so don't do that.
All that you ever wanted to know about batteries can be found at http://www.batteryuniversity.com/
We all have to recharge the battery every few days - just how long do you expect your battery to go without recharging? This is not a black and white Palm PDA which could go a couple weeks without hitting the charger. The battery is used even when it "sleeps", as you put it. In my case, my battery gets charged every night. I have to sleep anyway, so the BB gets plugged in at my nightstand. BTW, that is another thing with Lithium Ion. You can't overcharge them....
Hope this helps.