The FAQ seemed clear to me, and I like the 24 hour time limit. We can do 1 guild quest/day now, I wouldn't want that reduced. If you extend the time to 48 hours, you'll be waiting two days every time for inactives to time out. The guild quest has always been something that can be done in an hour or less with tagging lootspots or swapping resources amongst guild members. There's no reason for it to take 2 days.
This change will be a relief to guild leaders who are caught between a rock and a hard place with members who do their quests in the first few hours it pops up, and those that let it sit for a couple days. Now the guild quest completion is on a time limit, and if you choose not to do it, you're not holding everyone up. There will just be a new quest every day.
I would like, as a guild leader, to be able to keep track of who is not completing their quests, if it is a drag on the guild coins earned over a period of time. I'm sure the 'cost' of not having a member complete their quest will be made available with the Dev's Diary.
Like the stacking feature, this change will be welcomed - the sooner, the better for both of them.