I'm trying to understand how to use duration metrics. If we would like to measure the time it takes between a signup to the site and conversion (let's say we have a timestamp for both), how should the "duration" type metric SQL (default example below) be structured? Should we calculate the difference between the two timestamps and put it under "duration" ?

SELECT
  user_id as user_id,
  anonymous_id as anonymous_id,
  duration as value,
  received_at as timestamp
FROM
  sessions

The value column should be number of seconds. So in your case you would join signup and conversion tables and select the date diff in seconds. We should change that default example to

duration_in_seconds as value

to make that more clear.