Six or seven tasks of each color, on average, is a good starter mark to aim for. This amounts to a total of 1,440,000 to 2,340,000 total gold donated. While this may sound intimidating, in a full 30 person guild, this is average of 48k to 78k per person, and while that might even sound intimidating, a guild with extremely low contribution minimums of 20-30k will generally hit this unless literally everyone is coasting at the minimum range, or you have non-contributors, or it has very high turn over, because by the time you finish leveling your kingdoms it is trivial to put in ten times that much. The gold keys from the brown task alone refund about 10k to 11k per member and spread among 30 members, the first 5 tasks on brown return more gold than is put in, 6 about even, and 7 only very slightly less.
The key to finding a guild is to not join one of the random open invite ones, but instead talk to an actual person (on the formus, in chat) and get an invite, and find one with close to 30 active members. The less dead weight you are pulling, the better, even if there is huge imbalance between the contributions of the players at the various levels of progression, many people are fine with this as long as everyone is an active contributor in some way. Also, try to avoid guilds that only hit low 2-3 mil mark on total donations but insist on putting it all in one task, as they are only hurting themselves as the tasks have extremely diminishing returns on rewards and the temporary skill bonuses are categorically not worth it if you have to reach this hard, since you are losing a ton of rewards doing so that you will not recoup in extra gold by simply having higher stats for a week.