The publication date -according to Simon & Schuster- is May 6th, so this means that the cover should be out sometime during the winter. (If you count back 6 months from May, it is November. So expect to see a cover around that time!)
“@oscartcvz: " When will we see the CoHF cover?” Usually it's about six months before publication. So in winter.
— Cassandra Clare (@cassieclare) July 25, 2013