Some actors have assistants, and some have people who just answer their fan mail, in which case you won't have to wait long for a response. Some don't have that kind of help, which means you have to wait until they get around to answering it themselves.
The best thing to do is to send a self addressed stamped envelope, the size of whatever you are expecting to have returned to you. The easier you make it for them, the more likely they are to respond.
Have I ever shown you my Shattered Dreams box? It's in my Disappointment Closet. - Marge Simpson