When do you forgive a cheater?

I have been in a monogamous relationship with my boyfriend for 4.5 years. For the last year, we have been living together. He recently left on a business trip and called last night to confess that he had a one night stand two nights ago. He was upset, took full responsibility for the situation and was completely remorseful. When do you forgive a cheater? When do you leave? How do you forgive? What is the process?