If all you need is a number of minutes, then it's fairly easy. When you
subtract a date from another date (eg sysdate - flush_stamp) you obtain
a (fractional) number of days. Multiply the result by 1440 to get it in
minutes (it will required and additional TRUNC() or ROUND() because you
still have seconds hanging around) or by 86400 to get seconds.
I am trying to find the "number of minutes" between "sysdate" and "flush_stamp".