Detect how long the mouse is clicked

In my game

local Sec
Sec = 0
while true do
Local event, x, y = os.pullEvent("mouse_clicked")
If event == "mouse_clicked" and x < 3 or event == "mouse_drag" and x < 3 then
Sec = Sec + 1
sleep(1)
end
end

how do I detect how long the mouse is HELD down

It's only possible as of CC 1.74, thanks to the introduction of the mouse_up event. By checking os.clock() when a mouse_click (not "mouse_clicked"!) event is pulled, and comparing that value to os.clock() when mouse_up is pulled, you can determine the amount of time for which the button was held.

That said, depending on the nature of your game, you might simply be able to measure the amount of time between one mouse_click event and another mouse_click event.