Reasonable $child_timeout values (pre-queue)

[Michael Orlitzky]

(I just got done switching from post-queue to a pre-queue setup.)

Is ~45 seconds still the accepted "reasonable" value for $child_timeout
in a pre-queue setup?

And, how does its value interact with Postfix's smtp_data_done_timeout?
Mark once posted that $child_timeout should be less than
$smtp_data_done_timeout, but does it matter how much less, or to what we
set $smtp_data_done_timeout?