How can you detect if a player has held down a key for a period of time?

You can do it with this code. I also included the mouse key in case you need it. The script MUST be a local script, otherwise, it won't work !

Player = game.Players.LocalPlayer
Mouse = Player:GetMouse()


-- For Mouse button (Left Click)
MouseHoldTime = 0
MouseHolding = false
MouseTimeToHold = 10 -- Change this to the number of seconds that the player must be holding the mouse to call the desired function.

Mouse.Button1Down:connect(function()
	MouseHolding = true
	repeat
		wait(1)
		if MouseHolding == true then
			MouseHoldTime = MouseHoldTime +1
			if MouseHoldTime == MouseTimeToHold then
				-- Stuff
				print("Holded the mouse for "..MouseTimeToHold.." seconds.") -- This is an example.
			end
		end
	until MouseHolding == false
end)

Mouse.Button1Up:connect(function()
	MouseHolding = false
	print("Holded the mouse for "..MouseHoldTime.." seconds.")
	MouseHoldTime = 0
end)


-- For Key button (Key Pressed)
KeyHoldTime = 0
KeyHolding = false
KeyToHold = "c" -- Change this to the key that the player must be holding to launch the timer.
KeyTimeToHold = 10 -- Change this to the number of seconds that the player must be holding the key to call the desired function.

Mouse.KeyDown:connect(function(Key)
	if Key == KeyToHold then
		KeyHolding = true
		repeat
			wait(1)
			if KeyHolding == true then
				KeyHoldTime = KeyHoldTime +1
				if KeyHoldTime == KeyTimeToHold then
					-- Stuff
					print("Holded the "..KeyToHold.." key for "..KeyTimeToHold.." seconds.") -- This is an example.
				end
			end
		until KeyHolding == false
	end
end)

Mouse.KeyUp:connect(function(Key)
	if Key == KeyToHold then
		KeyHolding = false
		print("Holded the "..KeyToHold.." key for "..KeyHoldTime.." seconds.")
		KeyHoldTime = 0
	end
end)