I'd make some adjustments for accomplishing this in the npc/guild/agit_template.txt file.
Just before the end; of the OnStartArena label I'd add an initnpctimer;
In the OnAgitBreak, before any of the sleeps is executed, I'd put a stopnpctimer for making the timer stop.
Then, still inside that NPC, I'd make a new label like this:
OnTimer900000: // 900000 = 15minutes in milliseconds
//Here goes the prize, you should attach the script to the guildmaster or the guildmember(s) you'd like to get the prize, it's up to you
stopnpctimer;
initnpctimer;
end;
Some similar modifications should be made to npc/guild2/agit_main_se just if you use WoE SE, but I'm not that familiar with it and can't check from my cellphone.
Sorry for the vague response, I'm on my cellphone.
