a lightbulb vape? sounds interesting...

Anyway, from my experience with commercial vapes, you can keep hitting the herb as long as you're still getting smoke out of it. In theory, weed vaped 100% efficiently should have no THC, but there's probably some left in practice unless you really toast it. My friend who has a vape and uses it almost exclusively saves his vaped weed in a ziploc for emergencies.