For me, it's typically 50% direct-to-visitor's-profile, 50% visitor conversation. The conversation is helpful for reminding me of what we were talking about before; if I can remember the topic offhead, usually I just reply straight on the user's profile. I try replying to VMs as soon as possible unless something comes up or I have to plan out some long VM response (which fluctuates from 20% - 85% of my VMs, it seems >_>). The longer I wait to respond, the more difficult it is to jump right back into conversation, and I hate delaying responses to VM questions...