How long would this decay take?

Radioactive decay is a quantum mechanical process, that is it deals only in the realm of probabilities, not certainties. Therefore it's impossible to say when a sample will completely decay. At best you can only estimate the probability a certain sample will completely decay within a certain time period. Even a single atom of an unstable isotope that has a well-known half life has a finite probability of surviving billions of years. So your question has no answer.