When Airbnb assured employees last month that it aimed to go public before their stock grants expired, the company became the latest example of a private tech firm grappling with a once unheard-of problem.

Tech companies are staying private longer than ever, but most employee stock grants aren’t designed to last quite so long. Stock options typically expire after 10 years due to tax rules. Another type of stock compensation among larger Silicon Valley firms, known as restricted stock units, usually expires after five to seven years. That was once plenty of time for a startup to go public or get acquired, but increasingly, it isn’t enough.