Hyphens are used in the following instances:
With numbers over twenty:
We ate twenty-two pizzas.
She owned forty-seven cats.
I only had ninety-four cents in my pocket.
With compound nouns:
His sister-in-law was staying with us.
Their child is a four-year-old.
That coffee was a good pick-me-up.
With compound adjectives:
The check-in time is not until this afternoon.
That was a once-in-a-lifetime vacation!
Their house had a two-car garage.