This has become bit of an issue in our guild and probably in many other guilds too: we have plenty of people who play every day, and we have also plenty of people who play pretty much just during the weekend. And we like all of the players. However, guild quest taking constantly 6-7 days to complete is causing major unhappiness among the people. The active players are doing majority of the 'work' towards the quests, wait for days to get the rewards while most of the inactives just show up one day a week, collect the reward and go away again for a week.
So, is the purpose of guild quests to force us to kick all the weekend players to a separate guild?
Here's a perfect solution: swap the rewards around. For completing your personal part of the guild quest, reward 105 guild coins. If the whole guild completes the quest, everyone gets another 15 coins.
How does that solve everything you may ask? Now we can give everyone a day to complete the quest, get their 105 coins... and if it looks like the guild quest won't be completed, cancel it. No +15 reward today. The active players will be lot more happy getting 105 coins every day than 120 per week. And the casual weekend players are no longer harassed for being too inactive (which is due to work, family, having a life, nothing that they deserve to be harassed for).