There's no exact translation in English. It's kind of like "quite" "indeed" or "very much". But the French use "bien" a lot more than we would use any of these words in English. It's just to add more emphasis really. "It was quite a long time ago" 

Adding "bien" more formal too I would say. I'm sure you've come across "j'aime bien..." je veux..." "merci bien" where adding "bien" makes the verb that bit more polite. 

Thank you very much Niall :)