i think anyone who sets out to self-teach within a field like seo (ie only mid level complexity) will make reasonable swift progress initially.
marketing is much like this as it has maybe only 10 key things to learn to be `useful` to most businesses. However, nobody self-taught will have the skills to get a job in an agency working for coca cola worldwide (much less picking up a contract for them yourself) and this will be true of seo too.
to be an expert at anything takes much longer and requires a very `long tail` of minor peices of skill and nuggets of knowledge. these take a long time to aquire if you self-teach. and some may be impossible to aquire if they are exclusive to a small group of experts.
therefore the best and quickest way to develop in a field such as seo would be to work with and be trained by existing experts (single or in an agency) and then set up on your own later.
if you rely on only yourself to develop knowledge you choose a very slow route and a very narrow range of experience
"A self-taught man usually has a poor teacher and a worse student. "
Henny Youngman