You’re probably going to get a wide variety of answers to this but I think the most popular answer will be that there really isn’t one.
My GP had been telling me for 30 years (during my yearly physical) that I had a goiter and “One of these days, that thing is going to go kaplooey on you!” He would run blood work every year and send me for scans every few years but I was always in “normal” range. It finally went Kaplooey 7 years ago and yes, my symptoms were much worse for the few months before that. I went into remission a couple years ago but started feeling lousy in January. I thought it was because I had lots of stressful things going on but, come to find out, I was out of remission.
Now that I’ve had a TT and have been through several different levels in the past 8 weeks I’m getting really good at feeling when I’m in range and when I’m not and hopefully won’t have to suffer as long as I used to before getting treatment. Also, now that I have experienced what I’m SUPPOSED to feel like at a normal level, I really think this thyroid has affected me negatively for at least 20 years. So many of the things I’ve felt (bad ones) most of my life were gone during the few weeks recently when I felt good.
So in summary, I think you can suffer with symptoms for decades, and some people may be lucky enough to catch it in months. I guess it depends on how severely it hits you when it hits.