How can I setup Emacs >= 24.4 so that it saves at least the current file I'm working on automatically, say, each 5 seconds, so that I don't have to press
C-x C-s each time to save my changes?
You probably do not want it to save every N sec but to do that only after Emacs has been idle for N sec.
To do that, customize user option
auto-save-timeout to the number of seconds, and turn on
See also the Elisp manual, node Auto-Saving, which details the user options and functions used for auto-saving.