Since I can remember. It was my 'first love', and probably my single biggest hobby/interest even still.
When I was three, I was playing with my friends, stripped off most of my clothes, and used my grandmother's markers to color my skin green and draw question marks on myself. My mother was not pleased.
So Batman's been all-encompassing for me for a while. I grew up on the '60s show and Superfriends, and have stayed with it in every incarnation ever since - comics, movies, TV, figures, etc.