if you count the PC-98 games 1996 (first showing of TH1) 1997 (release of TH1) as well has the second game also released the same year which starting the shooting game aspect.
1996-2014 18 years of frilly dressed little girls.
well 19 since according to ZUN he started and completed TH1 in 1995 and just waited 2 years before releasing to the general public.