Hi @Shaz51, interesting question. I've tried to tackle it from a few different perspectives but it's hard to be definitive. Firstly, anecdotally from my client work, I know people who have been on anti-depressant (AD) medications for over a decade seemingly without negative effects. However, a decade may not be the time-frame you have in mind. It's possible that there could be health consequences farther down the track.
I also checked in with members of the Help Centre team who are currently studying to check what's being taught and there doesn't seem to be much information out there. I also did a quick search (as you've got me interested now!) and came back with some mixed results.
There's a strong anti-medication group who feel that we need to be addressing the cause rather than simply treating the symptoms with medication. This group believe that there could be some longer-term effects and there's some information online about that. On the other hand, others recommend weighing up the possible end result of untreated depression, which can include suicide, and so think that possible risks are worthwhile.
In my view, AD medication is ideally simply a bridge which allows people to make the changes which support their long-term mental health without meds. Reducing stress and managing it better, tackling problematic employment, finances or relationships, changing habitual negative/self-critical thought patterns, being more mindful (and focusing less on the past and the future), exercising, the right mix of social activities etc.
With chronic depression all those things seem extremely difficult to do and maintain. So medication can make them more possible. But once enough of them are in place, it might be possible to start coming off meds. It's all down to the individual though. I would never want anyone to feel bad for continuing to use meds. But it's good to start out seeing them as a temporary solution while you put more permanent ones in place.
Does this make sense Shaz? Sorry that I can't be more definitive. I'd love anyone else who has information to chime in too.