When do you know for sure that enough is enough? It is applicable in both cases - good and bad. If you go by a purely numerical definition, good or bad enough could probably be defined as anything that is 'x' units above the halfway point. That way, if you spend even 49% of your time in a particular situation in a good way (and 51% of the time in a bad way), you would still end up with two additional units of bad. Would that mean it would be a good idea to cut your losses, ditch the situation and move on ahead? Of course not.
The problem with people and relationships is that you can't ever really quantify either. A fair number of people exist in situations where 90% of the time spent is practically living hell for them; but the other 10% is, according to them, absolute heaven, which redeems the otherwise lousy situation. Naturally, in such circumstances, the good is given a higher weightage than the bad. Is that a smart idea? Well, I don't know. Whatever that rocks your boat, as they say, even if I may not agree with it. Moreover, factors such as practical considerations, only serve to muddy the waters further.
The question actually covers a whole lot of things other than just relationships. For example, you know you are putting on weight, but you are not overweight yet. So you get a particular weight in mind that would quantify the state of being overweight or a state of lurching rapidly towards it. But where exactly is that point? Is that point somewhere you can easily climb down from, like a minor flirtation over the lower limit or is it the minimum possible gap towards the upper limit, from where the climb down is a long way off, but not quite over the upper limit?
These are interesting questions to ponder, but very real ones too. People face it every day and deal with it using different methods. Some morbidly overweigh the little good and sign up for a lifetime of unbearable suffering, others make well-judged and sensible decisions that hover around the 50-50 level, which is more or less failsafe, while other idiots like me look for the 90% good and 10% bad to make the call that it is actually enough. Strangely, it is probably the 90% good rule that is the most prejudicial, especially if the other party has the lousy fortune to present the bad 10% up front first.