How long does it take before an academic article can no longer be kept behind a paywall? Just a curiosity: after how many years does publications become public (cannot be kept behind a paywall anymore)?
Strictly speaking, the answer to your question is Never.  Even after the copyright has expired, there is nothing to prevent a journal from keeping articles behind a paywall.  Many journals will make articles open access after a certain amount of time, and after the copyright has expired, other people may freely distribute public domain articles.  However, there are some journals that nonetheless have paywalled archives going back to the nineteenth century.