nnmail-expiry-wait

nnmail-expiry-wait is a variable defined in `nnmail.el'.
Its value is

7

Documentation:
*Expirable articles that are older than this will be expired.
This variable can either be a number (which will be interpreted as a
number of days) -- this doesn't have to be an integer. This variable
can also be `immediate' and `never'.

You can customize this variable.