When we change the operator extension, how long does it take to take effect?

Usually the operator extension is set to my extension. When i went out of town last week, I changed it to another extension - and it took a couple of hours for it to take effect. Today, in trying to switch it back, it seems like it again is taking some time to take effect.

Is that normal? It seems like it should switch in less than an hour? We are also going to get to a point where we are going to want to transfer it to someone after hours...but its not going to work if it takes hours to switch...