A little from column "a" a little from column "b" actually.
Starting him where we did in history, it seemed likely that we'd make our way through the Depression and right around the time that he might be (in the normal course of events) thinking about hanging it up sometime, then WWII comes along and blows that out of the water. I've always figured if they live through that I'd let them retire.
Certain things I know with absolute certainty. One particular supervillian's final demise, for example.... Other things I make up as I go along.... The basic arc that would (assuming I get to finish it) take us through season 9, I already know... but there are lots of things yet to be discovered within that arc.