They track "production ready" with the following item in their tracker:

https://github.com/rethinkdb/rethinkdb/issues/1174

There are dozens of production projects built on top of RethinkDB. We're starting to work with customers to publish case studies about their infrastructures and applications.

As bjg pointed out, you can track what we internally consider production-ready (when we hit 2.0) here: https://github.com/rethinkdb/rethinkdb/issues/1174 (although RethinkDB is already really good for doing real work, and we support all our users).