This file defines how long your server will attempt to process or deliver an email before bouncing it to the originating sender defined in the header of the email message.
The value in this file must be in seconds.
The default for Qmail-Toaster is 86400 (one day).
man qmail-send excerpt:
Number of seconds a message can stay in the queue. Default: 604800 (one week). After this time expires, qmail-send will try the message once more, but it will treat any temporary delivery failures as permanent failures.