When someone asks to interview you. When you're asked to write a blog post, give a presentation, moderate a workshop, dance on the table in your bloomers. Say yes! Well, maybe not that last one, but you get the idea. When you are getting started and working to build those crucial 1000 true fans, there is no event or blog too small for your time.
Many authors will say that this takes away too much time from their writing. To this I say, "Hogwash". I realize that everyone's time is tight and we have to prioritize. But I also realize that most of us waste more time that we claim to have. Let's be honest, the amount of time it takes to write a decent guest post is an hour, maybe two tops. Are you telling me you don't fart away an hour each day checking Twitter, watching TV, or playing Candy Crush Saga?
The point here is that marketing doesn't need to cut into your writing time if you let it cut into you slacker time. I'm not suggesting you aren't allowed to have down time. I am suggesting that your down time should be the smallest slice of your day's pie chart when you are working to grow a small business (and as an author, you are your own small business).
When you reach the point where taking on another interview exceeds your limited down time and now begins to creep into your writing time, that's when you say no. Chances are, when that happens its because you've reached a level of popularity that everyone wants a piece of the action. That's when you are allowed to start being choosy about which interviews to grant and which conferences to attend.
Until then, say yes!