How much would you have to inhale to get contact high?

The "contact high" myth is not true. The buzz that people feel when they claim they are getting a contact high is actually the available oxygen supply in a small space, lets say your buddy's VW Jetta, being replaced with smoke. They are feeling a buzz from lack of oxygen to the brain. You will not get a contact high, it is chemically impossible. The smoke from the materials being smoked, and the smoke exhaled do not contain enough tetrahydrocannabinol to produce even the slightest effects on your system.