How do I set up a function that waits for an event to fire but also returns a bool if it times out?

What you could do is create a timer that counts down from 10 and is refreshed every time damage is taken, which would have to be tracked by a separate function.

I'll assume you're using a humanoid for this.

function waitforchange(shield, duration, humanoid)
	currentHealth = humanoid.Health
	local healthchangedevent = humanoid.HealthChanged:connect(function(health)
		if health < currentHealth then
			print("Shield damaged. Regeneration timer reset.")
			duration = 10
		end
		currentHealth = health
	end)
	while shield do
		while duration > 0 do
			wait(1)
			duration = duration - 1
			print(duration.." seconds until shield regenerates.")
		end
		repeat humanoid.Health = humanoid.Health+5 wait(.1) until humanoid.Health == humanoid.MaxHealth or duration > 0
	end
	print("Shield destroyed!")
	shield:Destroy()
	healthchangedevent:Disconnect()
end

This script was not actually tested so if you find any flaws please message me. And no, the answer was not really obvious.