Jumping off both of your posts -- at what point do they decide to release the sketch if everything stays status quo?
This would depend on why they're withholding it, which only they know (not enough detail/too vague, not wanting to tip their hats,..) however time being critical especially these initial hours and first couple of days, there's not much room for error.
Also it doesn't seem like the witness or witnesses in this case will be able to fine tune their description of the suspect so the sketch won't become more detailed. What they have now might be as good as it gets and the public might have to work with what they have.
It's a fine line for LE and I don't envy them, but respect the heck out of them.