As early as possible. The trick is to find one that conforms to your playstyle.
The least demanding of requirements long term is generally one that only expects gold donations (possibly with low amounts of seals), as this generally just requires some form of weekly activity to keep up with requirements. There are also some guilds out there that allow newer players to build up their kingdoms before switching to contributions, allowing you to get to a point where you can easily farm their required donation with just a few scattered hours of playtime throughout the week while still giving you the benefit of a lot of extra resources.
The rotating guild events (Invasions, Raids, and Guild Wars) give you a several daily, but you can choose to play them on later days of the same week. These can be quite difficult and time consuming (even into endgame), so you’ll probably want guilds that have this as entirely optional. This usually goes hand in hand with the above “casual” guilds.
Be aware that we about to enter a new update where we will have a new feature, pets, with their appearance in an event linked to a random troop that can be triggered in PvP by anybody in the guild and only lasts one hour. This sounds like exactly the kind of thing that you don’t want to worry about, so it would be best to join a guild that doesn’t care about pets much. Thus far, they look completely non-essential for the rest of the game. This one is tricky, since this is an entirely new activity hook for this game, so I’m unsure which direction a lot of guilds will take and some might even have to restructure. But there should still be plenty of “casual” guilds that don’t try to enforce any stricter requirements because of pets.
Note that you want a “casual” guild: not a mismanaged or dead one. You want one with most of the players contributing something on a weekly basis so you can too and everybody benefits. Use guild chat or the forums to find a guild, not the ingame guild search system. You can go with a “super casual” no-requirements guild that might have a few (or no) other active players and cut yourself out of any requirements, but you’ll get almost nothing for it. The best return on investment low stress environment is something that just requires some amount of weekly contributions.