Full explanation
No official "one trigger every N spins" figure exists for Le Zeus, and that is worth saying plainly because plenty of pages will invent one.
What we do know from how the game is built: Mystery symbols only land on reels 2, 3, 4 and 5, and they show up regularly rather than rarely. So small Mystery reveals are a normal part of the base game, not a jackpot event. The bigger, chain-building reveals that actually move your balance are much less common.
Because every spin is independent, the spacing is genuinely random. You will get clusters of reveals and then stretches with nothing. Averaging that into a single number would be misleading even if Hacksaw released the data, because the average hides how lumpy the real experience is.
If you want a feel for the rhythm before risking money, run the free demo for a few hundred spins. That tells you more about how often the feature shows up for you than any made-up interval ever could.
Small reveals versus meaningful ones
It helps to separate two things this question quietly bundles together. Minor Mystery reveals that produce a small win or a short respin are fairly routine, because Mystery symbols visit the centre reels often. The meaningful reveals, the ones that start a long sticky chain or drop a Pot of Gold on a loaded board, are far rarer.
So if you are timing the gaps yourself, decide which event you are counting. The frequent small reveals will give you a short average; the session-changing ones will give you a long, lumpy one. Lumping them into a single number is exactly why a published average would mislead more than it informs.
In practice, plan around stretches rather than averages. Mystery symbols visit the middle reels often enough that small reveals feel frequent, while the session-defining ones are rare and unevenly spaced. If you size your bankroll for the dry runs instead of the typical run, the quiet patches stop feeling like the game has turned against you.