Many years ago the term webmaster had a different definition than it does today. Before a webmaster was someone who took care of everything related to the website, server, software, design, etc. as you can see in this definition at Webopedia.com:
Definition of Webmaster
So if you go off of this definition it can take several years to become knowledgeable enough to handle all of these tasks, especially obtaining the knowledge of web servers and proper setup.
But, if you are defining webmaste as someone who designs websites and maintains them, this can be done in a rather short amount of time, say several months if you put all of your time into learning the necessary skills. Of course, the more time and effort you put into it the better you will become.
In regards to what a webmaster should charge here are a few sites that list salaries of various positions:
I hope this helps. I posted a calculator I created in a previous post if you would like to see how much you should charge, you can find it here: Web Designer's Hourly Calculator