Mine was both gradual and sudden. How? Tinnitus increased in iover the years as I continually habituated. Then a loud blast suddenly increased the decibel level of my T, elevated the hiss, added a high pitched tone to my left ear and a reverb effect. All previous habituation was cancelled out. Took four to five years to "see the light at the end of the tunnel".
Habituation is no longer how long can I go without noticing T, but how well can I cope day by day. I'm working at lengthening the time I'm able ignore T, which is very short right now. But has started to improve since the five year mark.