Hashimoto's is an autoimmune disease, which I'm sure you already know. Thyroid function
sometimes declines quickly, with Hashi's and
sometimes, it goes pretty fast. It's impossible to say.
What are your current lab results? You should be getting tested periodically, for Free T3, Free T4, as well as the TSH. Be sure to post reference ranges, along with results, since ranges vary lab to lab and must come from your own report.