I have been on both sides. I've been cheated on and I've cheated. In most cases, it's caused the breakup. When I was in high school, I cheated on one of my girlfriends and never told her. When she cheated on me, I broke up with her. How's THAT for immature and hypocritical.
I've been in my current relationship for over 6 years. Around year 3, we both cheated and didn't tell each other about it. I hated myself for repeating past mistakes. About a year later, she told me about her indiscretions and I told her about mine. (I don't know if I would have ever told her had she not told me; I was torn as to whether I should ever bring it up.) Hers were worse, which made me feel a little better.
We broke up for a few months.
I always thought that if somebody cheated on me, that would be it. I don't know why I didn't give up right then. Maybe because we had so much invested in the relationship. Maybe because I didn't want to be alone. Maybe because I really loved her.
But it wasn't so clear-cut. I had cheated because things hadn't been going right in our relationship at the time. She had cheated because we had been together since she was 18 and was feeling locked down. When she studied abroad for a month and had a chance for "freedom," she needed to taste it.
We talked about it in-depth and went to therapy for a while to see if we could work it all out and regain each other's trust. That was a really rough year. We both came out of it okay and with a better understanding of what it means to have infidelity enter a relationship. Things have been unbelievable for the past three years - we've gotten along great, even through our fights. And we're engaged, with the obvious understanding that this is a greater commitment, and what has happened before will never happen again.
So when you ask "how much is too much," it's not a black-or-white answer that I can give. It depends on your relationship, and whether you think it will happen again. It still stings to think about what happened, and I'd be lying if I didn't say I was still scared of what the future could bring - but she has always promised me that it would never happen again, and I've promised the same. That's the best we can go on, I think.