How do i check the time of how long something is set true or false?

You can use tick() to get the difference between the times

At the top of the script, you can do this:

local startTime

Then when you set the value to true, you can do this:

bool.Value = true
startTime = tick()

And then when you set it to false, you can check if the difference between the new tick and the start time is more than 10

bool.Value = false
local timeTaken = tick() - startTime
if timeTaken >= 10 then
	print("10 seconds later")
end

If you just wanted to do all this whenever the value is changed, it would look something like this:

local bool = --set to boolvalue
local startTime
bool.Changed:Connect(function(newVal)
	if newVal == true then
		startTime = tick()
	else
		local timeTaken = tick() - startTime
		if timeTaken >= 10 then
			print("More than 10 seconds")
		end
	end
end)