Yeah it varies from person-to-person and then from time-to-time. A few weeks back I was stuttering in my thoughts too and I thought that something was seriously wrong with me. This week I feel much better mental health wise and I'm not stuttering in my mind. When I worry about it too much, or get paranoid of my stutter, I notice that I sometimes get so nervous that It seems to me that I'm stuttering in my head too.