Speaking as an ex-googler, the 20% time is much more about encouraging side-projects. Time management is completely up to the employee - there is no set day for 20% projects (which leads to the internal joke that it's really 120% time).

Some people choose to make time for their side projects, and some don't. The real benefit to 20% time is fostering the environment where employees are encouraged to help other areas of the company or to step outside of their day to day responsibilities.

This also promotes teamwork across teams, which is really important when you have so many different products that need to feel cohesive.