If you have not yet done so, you should subcribe to Selling Manager (on the site where you normally list - eBay.com or eBay.ca).
Then, it is a simple matter of selecting your listings by bunch (I usually do not edit more than 50 per batch) and bulk edit "the handling time".
http://pages.ebay.com/help/sell/sellingmanager.html
It is quick and easy and almost foolproof. 😉
Good Luck.