You never stop learning HTML because its always changing, but you should be able to work comfortably with it quite quickly- read a couple of books on it. Make sure you spend time on other scriptsas well though. HTML isn't very useful on its own.
Good, up to date html/css book worth reading
http://www.amazon.co.uk/HTML-CSS-Standards-Solutions-Standardistas/dp/1430216069/ref=wl_it_dp_o?ie=UTF8&coliid=I2PA5W1TKQIQ42&colid=25USNIIKZEJQM