I am considering buying a new Panda 4x4. I understand that the car battery can be disconnected when I travel abroad. For how long can the battery be disconnected, and then reconnected, so that the engine will ignite immediately?
Any idea?

Without parasitic drain, the battery should be able to sit without losing a charge forat least half the shelf life if it's a new battery. At that point I would worry more about the fuel in the tank going bad if we're talking about 6 to 12 months. You could also buy a trickle charging battery tender and just leave it plugged in.

I would suggest you can leave it connected. The 'drain' from a modern car when turned off must be unmeasurably small... unless you are planning years away?

If trickle charged there are instructions in the handbook relating to not confusing the battery voltage monitor, used by the stop/start system.

EDIT: the owner's manual suggests you should disconnect if unused for over a month, but also explains how to prevent damage to the stop/start system: the negative lead has a quick release, operated by pressing a button. This removes the main negative wire, but leaves the voltage sensor (and a very small wire connected to it) attached to the negative pole of the battery. (see page 81 of the handbook)