Oh, you should reply to comments in whatever way you feel comfortable of course; it's your art, your journal. I just get such a thrill from comments, especially on old art, that I can't help but want to smish whomever was thoughtful enough to take the time to comment after all these months (years!). Same goes for responses to comments I leave, no matter how long it takes someone to answer, in my opinion it's never too late. I used to hesitate to leave feedback on old entries, but experiencing the joy of a notification of attention for my own old stuff, I decided to not hold back only because the fanwork I only just discovered was old.

And on the subject of rules; I agree with de_nugis; it's probably different for every challenge:)