There is no time frame. Isolation is a coping mechanism that some sufferers use when they are feeling overwhelmed, so as long as they feel overwhelmed, they isolate.
You aren't powerless as a supporter though. You are perfectly within your rights to set up boundaries about what you can and cannot tolerate in your relationship when it comes to these isolation periods. When and if he comes back around, and is feeling better, you can sit down and discuss these with him. However while he is isolating, it is not a good time to bring anything emotionally heavy to the table.
My sufferer is an isolator. He does get overwhelmed and needs to be totally alone at times. This is just how it works. He, however knows my boundaries when it comes to remaining in a relationship.
I cannot tolerate long periods of isolation with no contact. I cannot tolerate disappearing acts. He also knows that I will not do this for weeks on end. We are in a relationship, and it takes two people to have one. He also knows that if he breaks up with me, he better mean it, because you don't have to tell me anything twice. I cannot have my heart ripped out by being "dumped" every 6 months. I will not remain in a relationship if these things happen. I cannot impose "rules" on him, but I can be very clear about what I can or cannot do in a healthy relationship. If the relationship is not healthy, staying in it is not good for anybody, him especially.
After the first gut-wrenching isolation period, we had to make some compromises at a time when he was feeling better. I had to accept that he isolates. Period. It happens, and it's no cake walk for him either. I had to agree not to contact him or try and engage him in conversations when he needs his space. In return he agreed to warn me when he needs to isolate. He cannot just drop off the face of the planet. He also agreed to shoot me "I'm alive" texts every so often so I do not worry something bad happened to him. I agreed not to try and use these texts as a means to start a conversation until he is ready (ie he usually texts "hi" instead of "I'm alive").
It may sound cold, but it is a heck of a lot better than sitting around suffering with unhappiness.