How long do you have to smoke for it to visual changes in your appearance (skin, teeth, etc.)?

Varies: In most cases smoking can take years to influence changes to ones' visual appearance. As a dentist I have seen changes in one's mouth occur after a few weeks. In addition to the obvious staining in the teeth smoking can alter the oral cavity's microbe population to favor growth of bacteria that cause gum disease and cavities. In a nutshell, smokers tend to have a higher prevalence of oral disease.

Few months to a year: Normally it varies according to how sever and how much smoking you do. It starts showing on your teeth it gives it that yellowish luster that does not go away with just brushing alone . Facial changes comes later after years of smoking.

It varies: Tooth staining from smoking varies but in many people the yellow tars stain quickly and can be difficult to remove without using an abrasive toothpaste that removes surface enamel. For this and many health reasons (cancer, emphysema, stroke, bad breath, cost, etc.) you should try to stop and get help to do so if needed.