It's impossible to generalise and if you try you're in danger of getting an overly simplistic idea of how people behave. Take quiting smoking, some people find it quite easy even if they've been smoking a long time, others don't. Some have very stressful lives that drive them to keep smoking whereas others don't. Some have a peer group that predominately smokes making it harder. You see what I'm getting at? It's not sensible to try and frame such situations as general tasks that only require a set amount of willpower.