Saving pg_stat_bgwriter snapshots

In order for this data to really be interesting, you need to save periodic snapshots of it with an associated timestamp. It's easy to create a table for that purpose and put a first snapshot into it:

pgbench=# CREATE TABLE pg_stat_bgwriter_snapshot AS SELECT current_timestamp,* FROM pg_stat_bgwriter;
SELECT 1
pgbench=# INSERT INTO pg_stat_bgwriter_snapshot (SELECT current_timestamp,* FROM pg_stat_bgwriter);

Now you can wait until some time has passed, with a representative chunk of activity; at least an hour is recommended, preferably a day. Insert a second snapshot into that table:

pgbench=# INSERT INTO pg_stat_bgwriter_snapshot (SELECT current_timestamp,* FROM pg_stat_bgwriter);

And now it's possible to get a difference both in values and in elapsed time between these two. You might even add the above INSERT to a query scheduling regime using tools like cron or pgAgent. Having two snapshots lets you compute averages in units like bytes...