When should I add the garlic to a roasted leg of lamb?

There's no benefit to putting the garlic in as slices from a flavor point of view, it's the cooking that brings the garlic flavor out. I've found that no matter how you do it the garlic flavor pretty much stays with the garlic rather than spreading throughout the meat - you don't get some even garlic hum throughout, instead you get parts with intense garlic flavor, which if you like garlic is a good thing. The only way to get it more even is smaller garlic slices in many more slits, however loads of slits will lead to the meat drying out.

From a food safety point of view cutting the slits and putting garlic in them that long before cooking can introduce bacteria or botulism spores deep inside the meat, and the temperature you cook it to may not be enough to make the food safe. If you use this method it's prudent to wash the outside of the meat, the knife, the garlic and of course your hands before you do it to reduce this risk.