How can I calculate how long is a player doing an action?

This is a basic Anti-Cheat that covers basically everything, including health changes, jump changes, and speed changes.

Make this a SCRIPT and put it into PLAYERSCRIPTS

local Player = script.Parent
local Human = Player.Character:FindFirstChild("Humanoid")
Human.Changed:Connect(function()
	if Human.WalkSpeed > 16 then
		Player:Kick("Caught speed walking")
	elseif Human.JumpPower > 50 then
		Player:Kick("Caught changing jump power")
	elseif Human.Health > 100 then
		Player:Kick("Caught changing health")
	end
	wait(1)
end

And also, here is the script you requested: It stops a player from jumping more than once in five seconds. Put this one in PlayerScripts and make it a normal Script

local Player = script.Parent
local Human = Player.Character:FindFirstChild("Humanoid")
Human.Changed:Connect(function()
	if Human.Jump = true then
		Human.JumpPower = 0 
		wait(5)
		Human.JumpPower = 50
	end
end

Hope this helps!