Well, exercise is beneficial, but it gradually starts becoming less beneficial beyond a certain point. Just like having a glass of wine every night can be beneficial, but beyond a certain point, the more you drink, the less beneficial it is.
I would say there is no exact "point". Professional athletes for example pretty much nothing else all day long. However for a normal person, it starts being too much when it 1) affects your health (i.e. becoming more injury-prone, physical exhaustion, etc.) or 2) it adversely affects other areas of your life (i.e. interferes with relationships, etc.).
Hope that helps!