I found I needed to do it quickly after ejaculating or I lost the urge or desire to swallow any cum even if I wanted to please my Mistress.
Keep in mind that this is probably exactly the reason it's such an attractive thing for your Mistress to order you to do.
Dominant women get off on controlling the other party. The greater the control, the greater the enjoyment.
Thus, the more an action affirms that there is actual control in place, the greater the satisfaction from ordering the act and seeing it completed.
To put that into a perspective you might find easier to understand:
If you order somebody to eat chocolate, while they really like and enjoy eating chocolate, their obedience might be perfect when they eat the chocolate, but the experience is not fully satisfying, because they probably would have eaten the chocolate anyways, even if you didn't tell them to do so.
However, if instead of ordering them to eat chocolate you order them to eat something they hate -brussel sprouts for example, then their obedience to you is affirmed in a much bigger way, because if they eat the brussel sprouts anyways even though they hate it, you know that they're only doing so as a direct response to your command, which in turn show that you have a great degree of actual control over their actions.
Most submissives have a momentary lapse in their deepest desires for submissive actions right after they cum.
That's because a lot of their desire for submission is directly fueled by the fact that being submissive turns them on, creating a great desire for submission, because it increases their own arousal, which is a pleasant state.
However, after they cum there's less of a desire for continued arousal because their body needs to replenish some of the hormones, etc, before it can built up to that peek arousal again. Because of the lessened desire for arousal, there's also a lessened desire for submission.
Thus, if your Mistress orders you to do something right after you cum. Especially something that's difficult, or distasteful, and you obey, then it will increase her enjoyment from your obedience, because she knows that it comes purely from a desire to please her, instead of from your own desire to increase your own arousal through obedience.
If you just came, that means that she helped you to increase your arousal to such a level that you had an orgasm and are sexually satisfied from it.
Now is your chance to show your gratitude for that, and pay her back, by doing something that shows her that your obedience to her is genuine, and not just self-serving, and that she has actual control over you. Which in turn which increase her arousal and sexual satisfaction from playing with you.
I can be your whore
I am the dirt you created
I am your sinner
And your whore
But let me tell you something baby
You love me for everything you hate me for