How would I make a GUI disappear after a few seconds of inactivity?

So basically I have a feature in my game, you hit stuff, it breaks eventually or if its a mob it dies. I have used os.time() to attempt to make a gui disappear if you haven't hit anything for 10 seconds however it doesnt seem to be working. No errors at all. It just doesn't do what I thought it would do.

Any help in solving this problem would be greatly appreciated.

Here is my script by the way:

local rockHit = game:GetService("ReplicatedStorage"):WaitForChild("RockHit")
local lastUpdate = nil

rockHit.Event:Connect(function(p, t)
    local plr = game:GetService("Players"):GetPlayerFromCharacter(p.Parent.Parent)
    if lastUpdate ~= nil then
        if lastUpdate + 10 < os.time() then
            plr.PlayerGui:WaitForChild("ItemHealth").Enabled = false
        end
	end
	lastUpdate = os.time()
end)