How much cocaine must be consumed in order for it to show up in a nail drug screen 30 day's later?

Any?: There is no real answer to this question. It depends on the sensitivity of the lab. Unlike urine tests that have established cutoff values, hair and nails do not. They are limit of detection tests, which means that any amount found is positive. They have to collect the nail from the right location (depends on the growth rate of the nail) then liquify it and pass it through a mass spectrophotometer.