It depends on the person whose entry it is, and on what the subject is. If the person was looking for information and it wasn't time sensitive, it's never too late to give the info. If the person was looking for advice on a personal situation, but it's been a while and the situation hasn't come up again in their entries, I'd leave it be. After all, if I fight with my boyfriend and then we mend things, I don't want someone coming in a few days later and saying, "Oh wow, he was such a jerk!" or "don't you think you were overreacting?" If it's just an average post about what the person has been up to, I might feel free to comment a few days later with, "Oooh, I loved that movie too!" or whatever. If the post caused a big drama flare-up and that seems to have since died down, I'd stay well away from it, and would contact the person privately if I felt it important to discuss it with them. In general, 3-4 days is fine for me unless it was one of the exceptions I listed above.