In a blink of an eye という表現は時間が急に早く過ごして気づいた意味です。Blink は「瞬く」という意味です。例えば、My summer vacation was over in a blink of an eye.
Before I knew it というのは日本語で「気づく前に何か起こった」と言います。いつも文章の最初のところで利用します。例えば、Before I knew it, my 6 years at elementary school were over.
I spent six years at the elementary school but it felt so fast.
The six years I spent at the elementary school went by so quickly.
The six years at the elementary school felt so quick.
時間があっという間に過ぎて短く感じることを、"feels (（とても）soやvery/（信じられないくらい）unbelievably/incredibly/extremely) fast(または）quick"と言います。「小学校で過ごした６年間はあっという間に過ぎた」= The time(the six years) at my elementary went by so quickly." "My six years at the elementary school went by so fast."
「長くて短い」をわざわざそのまま言うのなら、"I spent as long as six years at the elementary school but it felt so fast." など。
アメリカ合衆国 
日本 
