Earlier I was an active member of game-debate.com. And I can bet you the site was getting the best traffic for its niche. So, from my past experience, I can tell you that a good traffic would be the one which comes from the organic sources. By organic traffic I mean that the visitors which come from search results and not directly by typing in the address or through social medias. As I have my own AdSense account, I got more revenue from organic traffic and very less from non-organic ones. So, if your site is getting traffic from the search results, then I guess a regular traffic of around 1000 visitors would definitely do good to your site. But on the other hand, if your blog is getting views from your social profiles then I guess it should be more than 10,000 visitors to make a decent earning.
Apart from this, there are a lot of other factors on which a good traffic can be estimated. But I will not discuss that here or this thread will end up with a very long discussion. I do agree with the views put forth by other members here.