Well, for a start, stop using conflicting terms. "negative" and "Pressure" are total opposites. It is Vacuum or it is Pressure.
As to the rest, I will give you your answer if you tell me how long this piece of string is.
Meaning you have not provided any useful information. There are so many variables it is impossible to advise from what you have supplied. Humidity, air temperature, metal temperature, Dew Point, flow rate to name just a few. Are you bagging this or just waving a hose end around? Do you have a spreader or concentrator nozzle?
Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.
Insanity: "doing the same thing over and over again and expecting different results." Albert Einstein