What is the max capacity of memcached bucket?

What is the default ttl of a key in memcached bucket?

Also, what happens when a bucket is full?

I created a 256MB RAM Bucket and tried to overload it by sending infinite set queries till error is thrown.
But to my surprise, it keep accepting inputs, but total items now fixed to 2.8M with 204M RAM used.

Meanwhile, when I tried to fetch the first key it says key not found. Moreover, I keep calling a specific key infinitely it doesnt get expired.

So how is it exactly working? Please guide.