I personally think that people are waiting until clans/guilds serve an actual purpose. We currently know very little about how guilds will be supported or how they will actually function in game. When people feel they need to join a clan for X reason, that's when it will be time. Right now the only thing a guild can do is plan around the little info that has been disclosed.
If you really want to start a guild early, I'd suggest spending some time creating an overview about your guild. Use the typical who, what, when, where, and why when thinking about it's makeup. Who is your guild, what do you do, when do you meet, where is the guild's headquarters (website, forum, chat,) and/or why is the guild being created (what is it's purpose.) Cover all of these major things and more and you should be off to a good start.
People want to see polish and professionalism when joining a clan/guild.
"War is glorious to those who have no experience of it"