How much of a carcinogen needs to be consumed to get cancer?

Unknown. The short answer is unknown. Exposure or consumption of carcinogens alone does not guarantee development of a cancer. Radon isdetected in 1 of 15 homes but not everyone gets lung cancer due to it; also not just the amount but duration of exposure is important. The safest approach is to read about risks, keep use of presrvatives and chemicals to the minimum possible.
No answer. There are too many carcinogens and many different routes of exposure. There is no one known dose that would be safe or cause cancer. Many carcinogens act synergistically, i.e.. The sum of two is much worse than either one at a given dose. The development of cancer also depends on the genetic makeup of the individual, nutrition, age and gender etc.