The spec says seconds:
http://tools.ietf.org/html/draft-ietf-oauth-v2-22#section-4.2.2
expires_in
OPTIONAL. The lifetime in seconds of the access token. For
example, the value "3600" denotes that the access token will
expire in one hour from the time the response was generated.
I agree with OP that it's careless for Google to not document this.