In order for differential reinforcement to be most effective, the desirable behavior should be reinforced:

In order for differential reinforcement to be most effective, the desirable behavior should be reinforced: immediately.

Explanation:

A reinforcement can encourage or discourage a certain behavior. For instance, if you are complimented by your parents when you get good grades, you may feel encouraged to get more good grades in the future. The compliments are a reinforcement. Differential reinforcement happens when only the desired or targeted behavior is reinforced. Other responses or behavior are not reinforced whatsoever. However, for differential reinforcement to work, it has to be done immediately, otherwise the association between reinforcement and behavior will not be so strong.