This comment could mean two things. Benjamin might be using it to dodge more "politically incorrect" questions at hand; he seems to use the phrase every time he is asked what he things about Animal Farm and the way it is being run.
Another possibility is that Benjamin is insinuating that he is smart enough to keep quiet and avoid trouble for stating his opinion. Remember Napoleon's purges later on and what happened to Boxer for having defended Snowball in his role at the Battle of the Cowshed. Benjamin does indeed save his skin by remaining "zen" and politically neutral.
this sentence is actually a summarising of the whole book. it reflects Orwell's opinion, expressed more obviously in "1984", that things can be changed for the time being, but they will always go back to being bad, in a circle. in other words, the sentence means "if you will live as long as i do you will see how things go back to being bad" (the sentence being an answer to the question "aren't you happy now after the revolution?") this predicament is fulfilled in the end of the book: things have gone back to being bad, but Benjamin is one of the only animals that lived long inoff to witness this. other animals, like boxer, died peacefully, unaware of the inevitable circle and of things going back to being bad.