What day and time does the guild rep cap reset? I play on the US Wildhammer, a central time zone, realm. However, please answer this question so that it helps anyone from another realm.
The weekly cap resets along with Daily Quests, at 3 AM, server time, on Tuesday Morning for US servers, and at 3 AM server, Wednesday morning for EU servers.
Resets when all your realm resets occur, this is not the same moment on all realms.
It resets (for most characters) at the same time as the raid resets, etc, which tends to be 3 AM Tuesday morning for the U.S. or Tuesday night for Oceania. However I've heard reports that it varies for some characters, and they may reset at 3 AM Monday mornings (U.S. time) or other times during the week. However I haven't experienced that myself.
I also play on Wildhammer and Guild Rep resets at 4Am realm time, and so will your daily quests. Hope to see you on sometime!