Preferences > Account
So that you don't have to change the same setting every time you create an account, you can change some of the defaults on the accounts preferences page.
Many of the options are about account activation and will apply if you are creating accounts through the interface (bespoke self registration can use different settings):
All the values are whole numbers and any fractions will be rounded down.
If you manually set an account's expiry date further in the past than the automatic deletion point, it will be deleted within a day. This is fine if it is a deliberate act, but accidentally setting the expiry date as last year instead of next year will also cause the account to be automatically deleted.
Some agreements you have with publishers or federations may require you to trace a login back to a user for a period of time after it has happened. This cannot be done if the account has been deleted so you may need to set the automatic deletion period to allow for any agreements of this type.