It's good to have some meaningful defaults. For the cases of exception
it's nice to know why we decided to stray from the defaults.

service/prometheus.md

@@ -753,7 +753,9 @@ Here are some typical `for` delays we use, as a rule of thumb:
 - `1d`: daily consistency check. Examples: `PackagesPendingTooLong`
   (upgrades are supposed to run daily)
 
-Try to align yourself, but don't obsess over those.
+Try to align yourself, but don't obsess over those. If an alert is better suited
+to a `for` delay that differs from the above, simply add a comment to the alert
+to explain why the period is being used.
 
 ### Grouping