We have been lax about it, too, We have some that are hardwired, but the ones in the kitchen and our own bedroom are not so H and I first had to "argue" this morning about when they were last replaced before he agreed it probably was easier just to replace them (I wanted him to climb up and do it vs. me).
I think you're "supposed" to do it each time you change the clocks, however, I bet it's been way longer for us and it's the kind of thing that I normally only think about late at night when I'm trying to go to sleep.