I've had it for something like a decade. Mind you, I was always very protective about my hearing, using plugs during gigs, or over ear muffs when doing DIY and things like that.
For me it started once during the exams, I had really bad ringing in my ears. Went to a specialist, hearing range etc was perfect, apparently it was due to stress and lack of sleep.
And indeed, when the stress levels were reduced, so was the ringing. This went on for a year or 4, the ringing would come back during times of stress (exams, mostly), and would vanish again.
During that 4th year, one night I was struggling to sleep, the ringing started again...
And never went away after that.
It was really weird since I hadn't gone to any particular loud event, or wasn't exposed to any loud noises...
From time to time I really hate it, but most of the time, I'm used to it... It's like a part of you that you learn to live with, or that you HAVE to learn to live with.
So good luck and take care of your ears and yourself