In practice, for quantum mechanical systems, you get astronomical durations once the systems get even slightly complicated. It's possible to estimate and bound

In practice, for quantum mechanical systems, you get astronomical durations once the systems get even slightly complicated. It's possible to estimate and bound the recurrence time using the distribution of energy levels. Even the simplest chemical systems, like a single 10 atom molecule in vacuum, can have recurrences of years. A glass of water would eclipse the age of the universe, etc.