Javier,

That is easy to do.... but, you need a Wattmeter to gather the information.

If you have one, you can connect it in series with your charger after a timed flight. Then, when it's charged, read the mAH on the display that was put into the battery.

Now it's math... you have 2500 mAH to use, and you flew for 5 minutes, and let's say your charge put in 800 mAH. Assume you get 100% of your mAH rating (not real, but close).

So, you are using (on average) 800 mAH / 5 min. = 160 mAH per minute.

Now assume you get all your battery use to LVC, 2500 mAH / 160 mAH per min. = 15.6 minutes.

Now, to be conservative, take 80% of that = 12.5 minutes. Set your timer there and you should be fine. If you fly really hard, be a little more careful. You develop a feel for it, and you should stay up high and over your landing area when are close to your estimated time just in case.