Shizuka

Most of the time people average out at 4-5 lines from what I've seen but anything above that is certainly enough. It mostly depends on what you wanna do :P

I myself personally try to match my partner at least somewhat. I don't stress much about the number, just say what my character has to say!

Like Kalesh said, the average is from 3-5. I usually try to do at least 4 and a half, but when my rping partner is writing long posts I try to match the amount of content in my post with his/hers. Its up to you how many you write. (No one likes one liners though)

Rule wise 3 lines are the minimum requirements. But it's a good habit to match your partner.