A lot of time and effort, to make a guild takes no time at all you speak with GuildMaster NPC and you create one, simple as that.
But to effectively manage a successful guild, it could take weeks/months. (even close to 6+ months)
Depends what kind of guild you want,
If its an end game/leaderboard guild it will take the longest amount of time again depending on how much time your willing to add, as you will need.to gear up each and every member with the best possible gear, along with all the knowledge needed to get responsible/trustworthy officers to help manage the guild while offline. Then its their job to keep the guild in a 'peaceful' state which cuts out all the arguing and 'drama', also always on the look put for potential members. All while you will have to keep an eye on Officers checking the logs to see if anyone has been kicked out without your consent or just overall shenanigans.
If just a guild to chat to and have fun, then you could easily get it up and running in a week or less.
Momo Rocks!!!! Thanks for the Sig! Much love bro!