1. Flag has HP & Countdown Timer.
2. Countdown Timer = 0; Guild in control of the flag wins.
3. HP = 0; flag switches ownership and the countdown timer resets.
4. Guild in control of the flag gets preferable respawn points.
5. Flag HP is controlled by a statistical formula that is tuned to limit rewards to a sustainable level.
6. @Portha: You can continuously improve and eventually license the Flag HP formula (to e.g. SCM researchers, Economic Game Theorists). You can certainly get enough people to try to game the system to provide the data for preventing those strategies, and any and all demonstrable improvements to any basic formula you are capable of making can be profitable in more than one way if viewed from this lens.
The abstract formula is that Flag HP is set to a value equal to the expected Flag HP to use with the expectation that rewards are released at a rate that is sustainable in relation to the in-game economy.