Are we talking local/regional bands we play in? If that's the case, I would say one day after one member becomes an insufferable ass. If we're talking about big names, I think it's common that rock bands tend to decline in creativity...getting "stale" as @burntfrijoles put it. Jazz combos only get better because they learn to "read" each other. A trio that has been together for decades will create amazing music due of the highly relational nature of group improvisation. This probably applies to rock bands that go off-script as well, such as jam bands, as opposed to bands that just play the licks and parts as rehearsed and recorded...exactly the same, over and over.