According to the USPTO FAQ:
Most patent applications filed on or after November 29, 2000, will be
published 18 months after the filing date of the application, or any
earlier filing date relied upon under Title 35, United States Code.
Otherwise, all patent applications are maintained in the strictest
confidence until the patent is issued or the application is published.
(This was in response to the question, Is there any danger that the USPTO will give others information contained in my application while it is pending?)
Title 35 in the USC (which the above FAQ cites) has this to say:
Subject to paragraph (2), each application for a patent shall be
published, in accordance with procedures determined by the Director,
promptly after the expiration of a period of 18 months from the
earliest filing date for which a benefit is sought under this title.
At the request of the applicant, an application may be published
earlier than the end of such 18-month period.
Bottom line: 18 months after you file it, unless you request that it's published sooner.
There are some exceptions that provide that the application isn't published, but I don't think they are really important to your question (read: they were too dry for me to get through).