Read repair means that when a query is made against a given key, we perform a digest query against all the replicas of the key and push the most recent version to any out-of-date replicas. If a lower ConsistencyLevel than ALL was specified, this is done in the background after returning the data from the closest replica to the client; otherwise, it is done before returning the data.
This means that in almost all cases, at most the first instance of a query will return old data.
Range scans are not per-key and do not do read repair. (A range scan at CL > ONE *will* reconcile differences in replicas required to achive the given CL, but extra replicas are not compared in the background.)