Question

I’d like to write some vimscript that gets triggered if the vim session has been idle for a certain time. Is this possible?

Answer 1

:help CursorHold

When the user doesn't press a key for the time
specified with 'updatetime'.  Not re-triggered
until the user has pressed a key (i.e. doesn't
fire every 'updatetime' ms if you leave Vim to
make some coffee. :)  See |CursorHold-example|
for previewing tags.
This event is only triggered in Normal mode.
It is not triggered when waiting for a command
argument to be typed, or a movement after an
operator.
While recording the CursorHold event is not
triggered. |q|
Note: Interactive commands cannot be used for
this event.  There is no hit-enter prompt,
the screen is updated directly (when needed).
Note: In the future there will probably be
another option to set the time.
Hint: to force an update of the status lines
use: >
    :let &ro = &ro
{only on Amiga, Unix, Win32, MSDOS and all GUI
versions}
            *CursorHoldI*

So, you might try

let g:idle_counter = 0
function! Idle()
   echo "I am idle!!!! (" .  g:idle_counter . ")"

   let g:idle_counter = g:idle_counter + 1
endfunction

autocmd CursorHold * call Idle()

for a start.