How many years would a person have to be using a sunbed to develop skin cancer?

No way to tell: There is no way to tell. But, using it any, increase your risk of skin cancer, and themore you use it the more your risk could rise.

No set time.: Skin cancer's biggest single risk factor is sun exposure, combined with a person's skin tone. In the case of melanoma there are certainly other risk factors that aren't completely understood yet, as people get melanomas in non-sun exposed areas. One bad sunburn in your life increases the risk of developing skin cancer, more uv light exposure, the higher your risk, esp. Light skin/hair person.

No formula: Your risk of developing skin cancer from su exposure is the result of a complex combnation of several contributing factors such as your genetic predisposition, age, geographic location, severity and frequency of burns, complexion, immune status and several other factors.