As @C3roe says, the timestamp and timezone are not relevant to the rate.

For the completion rate:

1. Calculate the play time as a percentage of the runtime (episode length), and average that across the content_type.
2. (optional): Multiply that by 100 to get the percentage if required.
3. (optional): ROUND to N decimals places if required.

SELECT
    content_type,
    AVG(SUM([play_time(seconds)]) / SUM([Episode_length(seconds)]) * 100 AS avg_completion_rate
FROM
    Audience
GROUP BY
    content_type;