It all depends what you're doing. The biggest battery drainers are screen brightness and Airport. if you turn off Airport and set the brightness to 4 or less bars you should get an extra hour or so on your battery (at least, i do). Also, if you have BlueTooth on, that drains the battery. Besides that, it just depends on what apps you're running. If you have iTunes running the whole time, it will drain a lot of battery. if you just have Safari and iChat open, on the other hand, you'll get much more battery life.
As for your first question about closing it, when you put the computer to sleep, it doesn't turn off. It keeps the contents of the RAM alive so that it can turn on instantly. The sacrifice for this is that it uses around 1% of the battery per hour.