I set mine to about 30.
The way I look at this is your only going to send an email if the following is met which always has been the case.
It will send an email for any ad that is going to expire within 3 days and of course there is always checking to make sure we are only going to run cron based on the time you set and also it will only send one email per day.
Thats what I see when looking at the code.
Basically the time is $today. We grab the lastupdate time from the stats table and take the time right now and subtract the lastupdate time and the resulting number is timediff. If that number is 86400 which is one day then the if clause clause runs which grabs any ads expiring within 3 days and send an email.
Content visible to verified customers only.
Therefore looking at the code only one email can be send in any given day to a user.