after the first vapours, just stop lighting it and let it fill up, then pull till your lungs are full and re-light

you'll know youve burnt the weed when instead of misty light-grey vapour, you get white smoke rising right off the bud and that defeats the purpose of it...
also, on the topic of lightbulb vapourizers, ive had mine for about a month and I easily bake with .4ish - on the other hand, my coffee table vaporizer I bought for 100$ cant compete if what you want is to get high from as little as possible - my big vape works well with around a g, and that way, it'll get 3-4 people pretty high, but the lightbulb wins for a solo-bake