I use it every day and I don't come close to reaching my monthly limits. It's actually become one of my most used apps in recent months and I'd happily pay a monthly subscription for it - but the free plan is so generous I've never needed to.
Now, Google does also make money on ads, but the point remains that it really doesn't take a high conversion rate to get a freemium service to make money; as long as the cost per user is low enough, it can be beneficial to be very generous to your free users in order to attract a lot of people and get just a few more premium users.
You're right though. Someone ran the numbers on Spotify (the music streaming service). They charge $15 a month for ad-free listening or you can just use it free. Based on average listener profiles, it worked out at something as small as 1-2% of the userbase needing to pay to cover all the streaming costs involved (from what I recall).