If your concern is that you want to be fair, and not leave someone with a downvote for their placeholder answer as long as they clean it up quickly enough, there are two approaches to take:
If it is fully your intention to monitor the question and react to edits, don't wait. Comment explaining what you see lacking in the answer, downvote, and later check back to see if you should remove the downvote. Use a technique like favouriting, or bookmarking to be sure you come back.
If you kinda sorta mean to check back, but honestly you might or might not, then I would hold my fire until the 5 minute grace period has passed, and at that point vote or not, comment or not, and move on. If it's made much better someone else can update it for you, I suppose.
Me, I downvote and move on, grace period or not. They posted it, they stand behind it. If I comment, and if they @ me that they have fixed it, I might remove the downvote later but statistically, I can be pretty sure it's going to stay.