Hi Stuart
There doesn’t appear to be a global setting you can change, it seems you have to change it per item in Manage Inventory > Edit. On the Offer tab, change the Handling Time in Days field to what you want - if left blank the default is 1-2 days.
Alternatively download and use a category template file for your category to change all your items in one go. Just enter your SKU in the SKU column, enter a number in the leadtime-to-ship column and type PartialUpdate in the update-delete column. Save as a .txt tab delimited file and upload.
Sorry, even easier is if you download the Price inventory template file (only has 4 columns in it so much easier to use!) Enter your SKU and leadtime to ship, save as .txt tab delimited and upload.
Good luck!
Alison