To be honest: i never decide it's ready. I did some great reformulations on my perfume formula's over the years. Keep it as it is for months or even years, and deciding to substitute oils or aroma chemicals when newer or better arrive. Not always an improvement, but often it is. On the other hand, i've some formulations which i consider ready. I like them, friends like them, customers like them. So it's finished. I thought. And then comes that really nice aroma chemical on the market that can do wonders to an already perfect blend. ;-) Starting over all againÖ
But, hey, thats what i really like about perfuming: constantly improving!