Idle or in the off position? A lot depends. In the off position, I'd give it a week before it has to be on the charger. In the on position, it kind of depends on how bright you have the screen set, how much you are hitting the CPU and other factors. I'd say at least 4 hours, but could be a lot more.
If I've just got it sitting around in the on position, I leave it on the cradle. All batteries have a limited number of charge/discharge cycles before they have to be replaced, so I try not to discharge mine unless it is necessary.
If you need extra battery life, the expansion packs that allow one or two PC Cards also have an extended battery in them. If you don't need two PC Cards, the expansion pack that only holds one is much easier to hold and is more pleasing to the eye.
I appreciate that it is hard to tell. I was looking all over the compaq site for info about it and couldn't find a real claim to bat life any where. That's not surprising since the site doesn't appear to have been updated in a couple of months. Must be something to do with the hp merger. I couldn't even find a page about the newest model (3955)