Hi @Former-Member,
It's nice to see you here in Forum Land again.
I think it would probably vary from person to person as to how long it would take to totally screw yourself up with unhealthy eating, smoking and not sleeping much. For me, I know that eating too much junk food several days in a row will start to make me feel yuck very quickly, and a single bad night's sleep makes me struggle (which means I have huge empathy for those around here in Forum Land who super struggle with sleep). I've never smoked so I don't really know about that. I'm always amazed at the cost of cigarettes so I guess if nothing else, it would have the short-term impact of the expense.
Personally, I super like working at the things I can control (like eating properly and aiming for a good night's sleep). There is so much in my life that I CAN'T control, that it is super important to me to control the things I can in order to manage my emotions etc.