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There are two different forces at work here.
First, there is Tribunal decay. If you play for a long time without any Tribunal activity (no cases, and no more than the average player's number of reports) your punishment level will drop. This would completely eliminate the immediate risk of a permaban.
However, this takes more than just not getting punished, you need to avoid being reported above a certain (unknown) threshold. It also takes a long time. Nobody knows the exact specifications of punishment decay.
Second, if your punishment level does not decay, and you are brought before the Tribunal, and you are punished, Riot still must sign off on your permanent ban. However, circumstantial evidence suggests that they usually agree with the Tribunal in such cases.
So, the best answer I can give is "Not always, but usually."
I think the decay is at least 3-4 months. I had a 1 week ban and then about 4 months later a 3 day. And before that 3 day I had a warning.