Battery Life

Apple's documentation states that the battery of the Macbook Air performs just like the batteries of the Macbook Pro:

• The built-in battery of your MacBook, MacBook Pro or MacBook Air is designed to deliver up to 1000 full charge and discharge cycles before it reaches 80 percent of its original capacity.

• In addition, Adaptive Charging reduces the wear and tear on the battery giving it a lifespan of up to 5 years.

• During this lifespan the batteries will have a fast charge to 80% of the current battery capacity (2-3 hours). [source]

  

Battery Maintenance

• Do not leave your Macbook plugged in all the time. The battery juices need to move from time to time. It's recommended that you charge and discharge the battery at least once per month.

• Temperature: Keeping your Mac at room temperature (22°C/72°F) is ideal.

  

• Long Term Storage: Apple recommends that you store the battery with a 50% charge, if you are not planning on using it for at least half a year.

The innovation in the design of the battery of the Macbook Air was not about increasing capacity, but about the ability to manufactures them very thin. The batteries of the Macbook Air are located towards the thin end of the Macbook Air.

Does the real-world battery life scale linearly with it's capacity?

No, because it also depends on current draw. If you have a 10 watt hour battery, and you pull 10 watts from it, you will likely get less than 1 hour of time due to losses inside the battery.

If you instead pull 1 watt from it, you'll get closer to 10 hours of usage, because the losses are not as good at lower current draws.

As batteries age the losses become worse. The internal damage that occurs over time due to the charge/discharge cycle causes additional internal cell resistance. So a new battery will give you more of its rated capacity (and use less internally during use) than an old battery. The old battery will give you less of its rated capacity (and consume more internally than the new battery) than a new battery would.

Is this number a reliable indicator of present and future battery life?

It's an estimate, and now that the battery has higher internal losses it will depend much more greatly on how much power you're drawing. If you're using the processor and graphics chipset at capacity, then add wireless, bright screen, etc on top of that you probably won't be able to use the entire measured capacity.

Therefore it's not reliable, as it depends on your actual usage scenario.

If, instead, you've got it playing itunes with the lid closed, it will use a larger percentage of that capacity before it indicates depletion.

What kind of realistic usage am I likely to get from a new battery under Lion?

Unfortunately you're trying to compare Lion's consumption with your previous OS consumption. I can't answer that question, and even if I had the same model and recently changed the battery I have a different use case, so my experience wouldn't necessarily apply to use.

I can tell you that often one battery will degenerate more than the others in multi cell pack, and if discharged under controlled conditions you can get the full capacity of each individual cell, but under heavy discharge the bad cell is a weak link, and the whole battery pack will shut down prior to depleting the other good cells.

This means that there are several good reasons to replace your battery, and the most likely outcome is significantly increased battery life.

However there's no way to guarantee that due to your individual circumstances and usage. The tools you're using are ok, but without removing and testing each cell individually under a few different discharge conditions you can't know fully how the battery will compare to new batteries. So the tools can't be counted on to give you more than a rough estimate.

If I were in your situation, though, I'd expect to get approximately the same life out of a new battery under Lion as I got out of the new battery under the previous OS. The difference might be slightly greater, but still minimal.