There are no hard and fast rules that determine how long you should stay with a spouse when you discover they have been unfaithful. Some people upon finding out want to end the relationship immediately if not sooner. Their significant other violated the marriage contract so it's time for them to pay the penalty. Others hang in there for any number of reasons 

Is the relationship worth saving? Only you know the true answer to that question but before making that decision there are certain things to consider. 
  
1. Love

Do you still have those feelings for your spouse? If so how strong are they? Love could very well decide the future of the marriage relationship. However you do not want it to blind you to their wrongdoing. Your significant other might take that as get out of jail free card to keep doing what they are doing. 

2. A Real Apology

You find out confronted and they tell you they will never do it again. Now they might be sincere but that does not mean you should give them an answer right away. Step back and do some cold analyzing.  Don't let their words and emotions push you into making a hasty decision.

3. Observation

How are they reacting now that you have put your decision on hold?  It is not so much you want them to walk around with a sign telling the world about their infidelity but after being married to them for some time you want to see if there is a genuine want on their part to not only work things out but change their behavior pattern. Just a little observation without them noticing can tell you a lot.