Date model for commenting system stored in a key-value NoSQL DB?
In Cassandra you need to take a query-based modeling approach.  In your case, I would create a similarly-structured query table like this:

CREATE TABLE topicbyuser (
    topic_id text,
    user_id text,
    event_time timestamp,
    data text,
    PRIMARY KEY (user_id,event_time),
) WITH CLUSTERING ORDER BY (event_time DESC);


And then, if you wanted to query for the last 10 comments made by John, this would work:

SELECT * FROM topicbyuser WHERE user_id='John' LIMIT 10;


You could also add an ORDER BY, but since you have that in your column family definition, you shouldn't need it.  Also, you should note that this query table does not replace the original topic table.  It works in conjunction with it.