[Warning: This is a bit long-winded because I'm answering not just you, but also some earlier postings about the battery meter.]

It does show how long it can run (well, when you click on it), or rather, 2 numbers. One is if you just leave it on, but idling, the other if you execute something that needs CPU (e.g. surfing online and constantly paging through the browser).

The load the N800 presents on the battery varies with several orders of magnitude depending on what you do, that's why there are two figures that are so different (e.g. 10 days/6 hours). There could in principle have been any number of figures added ('3 hours if you do this, 6 if that, 14 if something else, 3 days if..'). This is what you get when you mix a highly optimized-for-power-saving CPU with a lithium battery.

The nature of a lithium battery is that voltage drops quickly if the current load goes above a certain level (you should try to watch a Palm T3 w/original battery and wi-fi card.. you could _watch_ it drop, second by second), then it comes back up again a while after the load goes away.

The net result is that it's very difficult for the battery application to come up with an accurate prediction about remaining time. It can look at the current drain of the moment and calculate remaining time from the capacity of the battery and voltage level of the moment (or some slightly different method), but a bit later the current drain may be 20 times higher or lower, and what then about the prediction? So, it gives you two values: The best case, and the worst case. Note that this is much easier to calculate on a laptop because the power saving modes are not at all as dramatic as on the N800, and even easier on a phone because most of them can be in just 2 modes only: Not using the radio (i.e. not calling), and using the radio (calling). Low drain, high drain (some ARM-based smartphones with wi-fi and PDA capacity leans more into the complicated direction, obviously).

That been said, I would like to have one or two additions to the meter: In addition to the predicted times it could have battery voltage and a simple percentage (where 100% is when it's at where the charging circuit cuts off, which is probably around 4.10 or 4.20 volts, and some sensible value down there somewhere for 0% (you would _never_ want to go that low though..). My Palm T3 starts to shut off certain applications somewhere below 15-20%, and it goes into sleep mode below that (configurable).

The 4-bar battery meter is a too crude voltage meter. The 3party app. FileZ on my T3 measures decimals to two digits. It's possible that this can be done on the N800 too, although it's not directly readable -- I dimly remember a discussion on the developer's list about a system value which just gives you a number, and someone would have to figure out how to convert that to volts.
With a specific voltage value you would eventually learn how to predict the remaining usage time based on your own usage patterns and experience. The 4-bar meter is too crude (not the same as inaccurate, which I first wrote here) for this.