Oral cancer screen. First get checked out by your dentist with an oral cancer screen. There are a variable of screening tests, which may be covered by your insurance. If the dentist feels an area is of concern, you may be referred to an oral surgeon for further evaluation. If possible, try to quit. The duration of chewing tobacco is not as important as getting evaluated and quitting. It is a known risk factor.
No set amount. You can use it once and that is enough. There is no way to predict how your soft tissues will react to the carcinogens. In any event, every time you use it, it increases your chances. It's almost like playing the lottery, but in a very bad way... You don't want to be the winner of this prize. Quit while you're ahead.