Multiple sources seem to use "2 years" as their rule of thumb. See, for example
https://www.ncrar.research.va.gov/Education/Documents/TinnitusDocuments/01_HenryPTM-HB_1-10.pdf
"A general guideline is that tinnitus of at least 12 months duration has a high likelihood of being a permanent condition (Dobie, 2004b). However, it also has been suggested that a person must have experienced tinnitus for at least two years before it should be considered permanent (Vernon, 1996)."
If your T keeps fading but is still audible 2 years after onset, there is no reason to think that it will stop fading after 2 years.
It will most likely continue fading.