Status responseStatus = cbData.response.getStatus();
... lots of logic ..
if (responseStatus.isOK()) {
.... stuff ...
}
else {
...
networkTime = cbData.response.getValue().elapsedMilis;
...
}

According to mattd@10gen.com, the implementation of the ReplicationCoordinatorImpl relies on the ability to retrieve the elapsedTimeMillis field of a RemoteCommandResponse even if the command failed. In that case the layout of RemoteCommandResponse could potentially be changed to:

struct RemoteCommandResponse {
StatusWith<BSONObj> data;
Milliseconds elapsedMillis;
}

or elapsedMillis could be stored directly in the RemoteCommandCallbackData instead.