Query for getting average engagement time per page from GA4 BigQuery data.

ga4_average_engagement_time_per_page.sql

SELECT
  page_location,
  SUM(engagement_time_seconds) AS engagement_time_seconds,
  SUM(page_views) AS page_views,
  SAFE_DIVIDE( SUM(engagement_time_seconds), SUM(page_views) ) AS average_engagement_time_per_page
FROM (
  SELECT
    (
    SELECT
      value.string_value
    FROM
      UNNEST (event_params)
    WHERE
      KEY = 'page_location' ) AS page_location,
    SUM((
      SELECT
        value.int_value
      FROM
        UNNEST(event_params)
      WHERE
        KEY = 'engagement_time_msec'))/1000 AS engagement_time_seconds,
    COUNT(
      CASE
        WHEN event_name = 'page_view' THEN 1
    END
      ) AS page_views
  FROM
    ## replace with your project & dataset
    `yourbigquery.analytics_12345678.events_*`
  WHERE
    _TABLE_SUFFIX BETWEEN '20230501'
    AND '20230531'
  GROUP BY
    page_location )
GROUP BY
  page_location