Adrienne, if you post vague jobs you get vague people responding to them.
Are these people expected to be merely hands or are they expected to be
creative, innovative, responsive, and committed to the organization?  Will
they eventually stay or are they intended to be temporary?  Seems these
questions need to be answered before the decision of when and how to post
gets made.  It's less of an etiquette issue than a practical one.

My $0.02 is to be specific in the ads about what the expectations are for
the people you need and you can get away with posting 6 months in advance.

Julia Moore
Indianapolis Art Center