Neem oil soap will repel most leaf eating insects because it makes the leaves taste terrible. You would use neem oil soap as a repellent in situations where, from past experience, you can anticipate a problem with a specific pest insect on a specific plant. If you had weevils on your azalea last year, you likely will have weevils on your azalea this year.
To use neem as a repellent you again spray the whole plant being sure to get all the top and bottom surfaces of all the leaves. While in many cases neem will repel for up to 30 days, it is wise to reapply it in about 3 weeks. You do not have to reapply neem after a rain. You can store unused portions of neem already mixed with water indefinitely.
It is important to remember that few pest insects hang around all season long. In most cases, they are a problem for only a few weeks and then they lay their eggs and die. So you do not have to apply neem for the entire season when you are using it as a repellent. You want to apply it a week or two before you expect the bad guy, and then use it until you don’t see the pest any more; seldom more than a month.
For more information about using neem as an insecticide to kill insects see the file Using Neem
For more information about a neem product see the file on Bonide\'s BonNeem