From the Craigslist FAQ:
You may post to one category and in one city, no more than once every 48 hours.
If you try to post something similar to an active post of yours on the site, you may get a blocked message
The first sentence makes it look simple: one post per category within 48 hours. However the second makes it seem like you're fine if you're not advertising the same thing. Is the limit once per category, or once per item per category?