No hours have been announced yet, the same way no dates have been announced yet.
From previous years experience, I can tell you that Halloween Horror Nights usually starts around 7pm & goes till 12 midnight on non-peak nights (weekdays & sundays) & 2 am on peak nights (fridays & saturdays)
This is just what has been done in years past tho, nothing is set in stone untill Universal officially announces it & releases the dates & times.
Hope that helps !