When should I spray for scale, with neem oil, on my cork tree?

The neem oil is the right thing to use. Just make sure it is below 90 degrees or the oil could burn the foliage. It is best to spray in the morning or evening. Make sure you read and follow the directions on the container.

Related Videos:
Horticultural Oils
Scale

Related Resources:
Armored (Hard) Scale - Trees and Shrubs
Soft Scales
Insect Control: Horticultural Oils