Basically, each cosmological decade is ten times the length of all the previous time put together.
The word "decade" does not always mean ten years, it can also mean a set or series of ten.
The universe is now a little over ten to the tenth power years old (ten billion, or 1 with ten zeros after it), so we are living in the tenth cosmological decade.
When the universe is 10 times older than it is now it will be in the eleventh cosmological decade(100 billion, or 1 with eleven zeros after it). Ten times older than that and it will be in the twelfth.