For me if I'm bored I'll kill the conversation with a response that shouldn't be replied to and leave. If I don't respond at all it doesn't automatically mean I'm not interested though.
It goes like this: Effort required + Excitement factor = directly relational to response time.
I'm forever forgetting about conversations I've started online, really totally forgetting, if I don't answer straight away after reading something I generally forget about it for weeks or even months, this covers PMs, wall messages, threads and even my blog. Hell I created a thread a few days ago that people responded to and I read (and found interesting) then forgot to answer...
Basically the more time and thought I have to put into a response the greater the chance you won't hear from me for weeks, writing long posts and/or in-depth posts isn't something I enjoy doing so I put off until the perfect time, the perfect time rarely comes, so I forget.
I don't like doing things in half measures, this often results in me not doing things and while it is true that the more excited I am about something the greater chance I'll deal with it straight away, my failure to respond doesn't always mean I'm not interested.