Like most things it depends on the circumstances.
Assuming the person was "caught" it's been said that women forgive easier than men. It also matters who their mate cheated with (a family member of the spouse, close friend, on going affair, one night stand, prostitute/escort, someone of the same sex while being in a hetrosexual relationship...etc)
Everyone has their own "deal breakers". For some people the thought of someone they love lying to them while having sex with others and possibly putting them at risk for STDs or bringing a child into the world is too much for them to overlook. They simply can't get the picture out of their mind nor stop thinking about how the cheater made a fool out of them.
Forgiveness is easier to offer when the person who was cheated on actually takes ownership in possibly neglecting their mate in some way (not having sex for several weeks at a time, verbal/mental or physical abuse, emotionally distant)
However even in situations where both people roll up their sleeves and work hard to get back on track there will be lots of challenges. The cheater must show contrition and be willing to live an "open book" lifestyle for as long as it takes their mate to regain trust. Their mate has to refrain from throwing the cheating incident in the cheater's face. A good therapist may be of help to get to the bottom of issues that led to the betrayal.
Last but not least the person who was betrayed has to be willing to tell their friends, family, co-workers, and whomever else that knows about the betrayal to "Shut up!" It's difficult enough to rebuild trust without other people placing doubts in your head over every tiny issue that arises. Some couple have actually been known to claim their relationships became stronger after the affair.