After how long does periodontitis appear, if one kept gingavitis untreated?

It depends: There are so many contributing factors to periodontal disease other than just gingivitis that it is different for every individual. For some the transition may be very rapid, while for others it may never change, and for others it may be a slow change. The best way to keep it in check is to see your dentist regularly, practice good oral hygiene every day, and maintaib good health.

Periodontitis under : Gingivitis is inflammation and breakdown of the gums or soft tissues but if left untreated the infection and inflammation progresses to the bone cementum and periodontal ligaments which make it very difficult to control since you can't clean easily below your gums and the bone will start being destroyed. This same bone holds your teeth and when too much bone loss occurs you lose teeth. Treat it now.