I'm on a similar plan with 10% (not at Google!). It's not really deadline sensitive. The 10% projects tend to be long-term so I can easily move them off my schedule for a while and spend more time on them after.

The whole thing is optional for us anyway and in fact needs to be approved every 3 years.

And I'm not really 100% occupied at work anyway. If I were, how would I fit in learning, networking etc?