How much plastic fumes must you inhale to have a dangerous effect on you?? l directly inhaled some last night without knowing it, and l feel a bit out of it right now. How much is considered dangerous? How worried should l be?

Not very concerned at all, unless this is an everyday thing for you.
Chronic exposure to any airway irritant is harmful to the normal function of your respiratory system. Anytime you're going to be exposed to something of that sort, you need to wear a mask. They can be bought very inexpensively at most auto parts retail stores.
God bless!