What do you have in mind? I can't think of a case where you'd want "X is typing" to persist more than a second or maybe two past the last-received message that affirmed X is, in fact, still typing. I can think of cases in which a short timeout might break down in bad ways (making the notice really jittery on high-latency connections, say) but I think those are probably just situations in which any "... is typing" message is simply gonna be bad in one way or another.