Seeing it back like that makes we wonder if I did the timezone conversion correct. According to siedlertools.de it should be around 03:00 (assuming CEST). At the moment that would be around 01:00 GMT, but without DST it would be around 02:00 GMT.
According to siedlertools.de it's 48 hours, however I think it's a little bit different, but haven't checked that.
After 24 hours your island stops production. If you island is not producing while the new Guild Quest becomes active, you won't get the guild quest
Examples:
If you have no waiting guild quests and you log out 15 minutes after the new guild quest becomes active, then your island would still be producing when the next guild quest becomes active, but you won't get the one that is handed out 24 hours later, since at that moment your island did stop it's production. (So here it's about 48 hours)
On the other hand, if you log out 15 minutes before you get a new guild quest, then you would get that guild quest, but not the one that is handed out 24 hours later. (So here it's about 24 hours)