[Exim] Is there a RFC or set standard for setting retry rule…

Is there some standard or RFC for setting the defaults of retry rules?

For example, the default that came with my one of my debian-based
exim.conf files retries up until 4 days passed since the first failed
delivery.

Where is this suggested "4 days" documented?

(My sendmail says the default timeout is five days.)

Thanks,

Jeremy C. Reed