How fast do feelings fade?

If a girl breaks up with a guy after almost a year does her love for you leave quickly or does she still have feelings that could make her miss you and change her mind? What needs to be done to try and re-kindle the emotions. The break-up was because I had been starting to doubt the relationship and it started pushing her away by arguing about it but as the saying goes, "you don't know what you have till it's gone!"
How fast do feelings fade?
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