I bought my first Apple product ever last week - an iPhone 3GS. Its very nice and sleek and fast and lovely but should the battery run down as much as it does?

Since last charge...
Useage: 40mins
Standby: 1 hour, 39 minutes

In reality I sent 3 text messages and played with an ap for about 5 minutes. I have the screen brightness turned all the way down, 3G is off, wifi and bluetooth are off. I'm already down to 86%

Is this normal? I have no point of reference with this being my first product.