If you're in it alone, I'd say 3 - 5 years.
- HTML5
- CSS3
- Javascript
- basics of image manipulation
- some kind of backend language (PHP, Python, Ruby, Node.js,...)
- some kind of database engine knowledge (SQL, NoSQL, both?)
- user experience planning, testing and execution
- extensive server administration
- different APIs
These are just the technical aspects, now count in the whole business side of things. 3-5 years. Probably more if you want to master all of the above.