If you've got something like that Drakla, then rather than estimate the time for the whole job; just estimate the time for one iteration of the loop, and then reset the time limit to that estimate at the start of the loop. Then you don't need to re-estimate times if the length of your mailing list changes.
foreach($address in $address_list)
{
set_time_limit(10); // Or whatever.
....
}