Breakup - Will they ever regret leaving you for someone else?

Many of you have been left for someone else, often after a long and good relationship. They meet someone else and suddenly they feel they would be so much happier, and they feel a strong connection, are madly in love. The long-term relationship is given up andyou are replaced in no time for someone else. I wanted to know from your experience and knowledge: what is the chance that they will ever regret giving up a long relationship for someone new? And IF they regret it, how long does it take - months, years? There is probably no science about this, but think you all have much experience, or knowledge....