My tinnitus also came "mysteriously". Well, I had an anxiety disorder at the time. On the Dutch tinnitus forum some say my tinnitus will go away once my system is recovered from the times of anxiety and only the part possibly caused by hearing loss would stay (which would be a tiny T as my hearing is almost perfect, also no hidden hearing loss as far as I can tell). But I doubt it. Some Dutch tinnitus clinic says there is nothing to do about tinnitus once you have it for two years, another source says it's chronic after six months.
I am past six months now and you know, I do not really care about whether it is permanent or not anymore. Early those 'deadlines' would only bring me fear. Now, the thought that I can habituate to this thing is most important. There WILL be a time that I no longer see tinnitus as a problem and I will no longer suffer from it. It will be a mere background sound which does not ask for any attention. For me, the sound may stay for ever.