Specific niche obviously. The only chance you have of ever getting a general niche off the ground is by word of mouth and Buzz. And good luck, beceause you'd need a hell of a good idea for that
I'd say it really depends on the niche...both in general and specifically. Though, if you write with knowledge and passion you should be able to get anything going...though, the more saturated the market for your subject, the more promoting you'll need to do.