- You want what you want when you want it. - This phrase is often used to express sympathy to someone who may or may not be getting what they want at a given time, saying "Yes, it's understandably frustrating that you can't get what you want right now, because you want what you want when you want it and if you get it any later than that you will get what you want, but you will still be perfectly justified in being less satisfied than if you got it when you originally wanted it." Taken as such you can of course use the phrase sincerely or ironically, perhaps the person is in no way justified in their dissatisfaction, or should just suck it up and take what they want when they can get it, but the explicit or inverted meaning of the phrase is clear depending on the sarcasm leveled.
The sideways meaning that prompted an Idiom Watch however is the implication hidden in a slightly different grammatical interpretation. What if you are the type of person who wants something when they want it, but often no longer wants it any time thereafter? What does this mean about you as a person? That your desires are entirely time-sensitive? Perhaps, which again could be a justifiable position in many cases (someone in need of a heart transplant could justifiably be said to want what they want when they want it). On the other hand, perhaps you are just the kind of person who has fleeting desires, and if you don't get what you want when you want it then the desire flees and you're not much worse for not having gotten something you wanted an hour ago.
A third option is that you have not fleeting, but rather powerful and fluctuating desires. The desire doesn't run from you when the clock strikes another hour passed, but it looks different than it did an hour ago. Maybe the person offering something to you then has made you a new proposal, the alternative they proposed an hour ago looks more desirable with an hour's thought, or you were just downright wrong in wanting the thing and could only learn it with reflection. Getting what you want when you want it then can even pose a risk of not get what you ultimately want.
A person who wants what they want when they want it then has three possible outcomes in descending order of satisfaction. If they have strong, temporal desires, they will often be dissapointed by delays. If they have fleeting desires, they can never experience lasting contentment. And if they have powerful but fluctuating desires, they will strongly favor things they wanted then at the expense of better things they could have gotten now.