The question was "how many years..." that'd be around 30 years.
Many have heard of or seen snail explosions but that experience has not been hands on.
That is not the case.
The pre snail explosion conditions always seem to have one thing in common, sadly neglectful. Snails in a reasonably well cared for tank will multiply and sustain a population relative to the environment without ever becoming a major problem. Multiple types of snails will lower the sustainable population for any one species. If you fear an unmanagable snail outbreak, I would respectfully suggest that you have a strong aversion to snails or you may want to consider reevaluating your aquarium care strategies.