How long were these things kept secret before being revealed to the Apostle?
These things were kept secret by God until the time He revealed them to the Apostle. No duration in years or ages is given; the context only states they remained hidden until that revelation to this Apostle. Nothing specifies how long the secret was withheld.
Les explains that the same God saw fit to keep all these things secret until they were revealed to this Apostle. In verse 10, the question is why God has now revealed this secret of the mystery: so that now unto the principalities and powers in the heavenlies might be known by the church the manifold wisdom of God. Les emphasizes that the church is the assembly of called-out believers—not merely seminarians or preachers, but every common, ordinary person in the pew. God wants every believer to study and understand the Scriptures, as Tyndale sought to put the Word of God into the hands of every plow boy in England. Les notes that nothing thrills him more than hearing someone say they disagree or get a little mad but have determined to start studying their Bible.