VERY good topic, Tiger.
My feeling, based mainly on how I see "most" people doing it, is that yes, IF you choose to show yourself, as Available, then you SHOULD respond within a minute or two maximum to any IM.
When I send an IM to a friend, I usually stop everything else and sit there waiting for / expecting the reply.
If you are NOT prepared to reply immediately, then that's exactly why most IM programs offer Statuses like "Away", "Busy", etc.
If I see you're Away I may still IM you, but I won't expect an instant answer.
But if I see you as Busy - then I personally won't even IM you at all until later, as it would be distracting for you.
And I think those broad "rules" are followed by most people I know online - though - many of us move in completely different circles of friends, I guess.