You can estimate for yourself when a besieged will take place, by checking the Besieged map. It's located under Region Info, right near the Conquest map ... or, you can just type /bmap in-game.
I posted a picture in this thread that will show you what I mean:
http://www.qcdn.org/showthread.php?t=29470
Notice, in the top left of the map, you can see the level of combat-readiness for each beastman army. In this particular case, they were all at Level 1 at the same time ... but it isn't usually that way.
Most of the time, you'll see them at Level 0, which means they aren't getting ready to attack at that particular moment. Hover over the location of that army on the map to see the level of their forces. It'll probably be somewhere between 50 and 90. If this is the case, you've got hours and hours before that particular army invades again.
Once the forces of any of the three armies reach 100, however, the rate of troop recruitment accelerates, to something I'd estimate to be roughly one unit every four or five minutes. Since our Besiegeds have been at Level 5 for a loooong time, you can pretty much count on them not actually advancing until they hit 150 (Level 5).
I just logged out of the game, and noticed that the Undead Swarm were at Level 1. That means that Besieged is probably at least another two hours off ... not half an hour. To make an estimate for yourself, just log in, check the map, time how quickly it takes them to gain one unit, then make a calculation for how long it should take the army to reach 150.
Once that happens, if you're in the Aht Urghan area, you'll get an "advancing" message. It is from THAT point you've probably got half-an-hour more.
To answer your main question, however ... no, it doesn't matter where you are in Al Zahbi. You'll be re-located by force once the invasion starts. Just make sure you aren't in Whitegate.