The 90-day rule for an IPO generally refers to the standard post-listing lock-up period restriction (which can range anywhere from 90 to 180 days or more) during which corporate insiders, early founders, venture capital backers, and pre-IPO employees are contractually prohibited from selling their restricted shares. Once this lock-up window expires, a massive influx of newly eligible shares can flood the open market, frequently causing downward pressure on the stock price.