What amount smoking would it take for your lungs to get bad?

Depends on what you: mean by "bad". There is evidence that there is some damage even after a single cigarette, but this will quickly heal. Permanent damage to the lungs - chronic bronchitis,emphysema,cancer - take generally decades of smoking. However, particularly in those starting very young, and those smoking very heavily (2-3 ppd and more), damage can occur much sooner, even in a few years.