when you make dua, when do you stop?
If what you asking for doesn't seem to come to light, virtually impossible, do you stop then?
Whosoever desires (with his deeds) the reward of the Hereafter, We give him increase in his reward, and whosoever desires the reward of this world (with his deeds), We give him thereof (what is written for him), and he has no portion in the Hereafter. (Qur'an 42:20)
Firstly, the way you're asking questions about Du'a imply that you need to learn a bit more about the way of asking Du'as.
For example, as the above verse says, you won't prefer asking Dua's for the worldly things.
Try reading Qur'an and getting started with Dua's(Qur'anic Dua's) I hope you'll find that a lot helpful.
Indeed, the righteous will be among gardens and springs, Accepting what their Lord has given them. Indeed, they were before that doers of good. They used to sleep but little of the night [invoking their Lord (Allah) and praying, with fear and hope], And in the hours before dawn they would ask forgiveness, ..... (Qur'an 51:15-18)
As the above verses say, you need to learn to know the highly recommended times for making Du'a and asking forgiveness and such things. You get that by reading Qur'an and related Hadith.
To your question, you stop making Du'a the moment you feel "Ah! Allah might have heard me by now, and is very likely to grant me what I've asked."
May the creator guide us all.
Allah can do everything so ask for most everytime. Even ask for heaven of Firdevs (the highest floor of heavens)