I want to know that, if istekhara comes negative then how much time required to see the istikhara again. Actually I like a girl who is the niece of my friend, We like each other very much, and fallen in love but did not go beyond the limits of shariyah, both the families are also agree to make the relation, in this concern, first of all my friend brought proposal of her niece to me, then the same night he saw a nightmare. Then he discussed to me and I said go through Istikhara and made discussions with your senior members of family. Then they did Istekhara in which they found the negative results. Not exactly they got the points of refusal but only they had some hints like bad smell, or fireball. Please let me know how much time is required to see the Istikhara again as already its been over 100 days passed. I love her very much and I don’t want to loose her and also refer me some dua’s or wazaif which can make difference, as Allah’s says only dua can change one’s fate nothing else. So please do something for me.

There is no objection to making istikhara again how much youlike in the times and places and means you`d normally be allowed to pray any nawaafil. Of course not. Du`aa is an `ibaadah, and Solaah, which are open in the sense that you don`t have an upper limit that now you can`t pray anymore.
In Hisn Al-Muslim you can find good du`aas. Please refer to it. When you are fasting, your du`aa is most easily accepted, espicially in the mosque between athaan and iqaamah, so do that in sha Allaah.
Her parents should understand something about La ilaha ill-Allaah, which many people among this ummah are unfortunately ignorant of, and that is that actually we are not allowed to believe in bad omens. There`s a chapter about this for example in Kitaab At-TawHeed:

http://islamic-knowledge.com/Books_Articles/Kitaab_At-Tawheed__Sh_Muhammad_At_Tamimi.pdf

Please study it, because La ilaha ill-Allaah is more important than marriage.
So, anyway it`s not that you see a good dream or a bad dream, because dreams can be either from Allaah, or shaitaan, or from your own sub-consious mind. Only the dreams of ANBIYAA -as- were revelation from Allaah. What is there that you say in the du`aa of al-istikhaarah? You ask Him... "if You know this affair (which YOU HAVE DECIDED to do! Get it?) is better for me...then make it happen to me, and ease it for me, and then bless me regarding it. And if you know that (this matter) is worse for me...then take it away from me and me away from it, and make it happen to me what is better whatever it is, and then bless me regarding it!" This is what you ask for, isn`t it? You`re not asking if it`s good, then show me a good dream, and if it`s bad then give me a nightmare! Man, get a grip on yourself. Look at what you have asked, and what you have got! You still love the woman, don`t you? So you haven`t been guided to something else and become pleased with it? If you have made istikhara, and this girl is still in your mind, then it means the answer is you should marry her. If you haven`t made istikhara, then you should, because it`s your affair, and not her parents. And after YOU make istikhara about the DECICION you HAVE MADE, and you try to work for it, THEN, and only then will you see the answer. AND IF Allaah turns it away, THEN it`s a "not this one", and you WILL find better, and you WILL be pleased. You have to trust upon Allaah, but that doesn`t mean Allaah is going to live your life on your behalf. Get it? Don`t wait for a dream, boy. Take action, as a man should, because men marry, and boys play.
You have made your decision, and she has, and families have agreed, so marry. What`s stopping you, except belief in evil omens, which is shirk, wAllaahi, and I`m giving you the sufficient evidence:

Allah (swt ) says:
" And when good came to them, they said: "This is due to us." But when evil befell them,
they ascribed it to omens connected with Moosa and those with him! Verily, in truth the
omens of evil are theirs in Allah's sight, but most of them understand not!" (Qur'an 7:131)
Allah (swt ), Most High, says:

" They said: "Your evil omens are with yourselves." If you are admonished [do you deem it
An evil omen?] No! But you are a people transgressing all bounds" (Qur'an 36:19)

Imam Ahmad reports, on the authority of Ibn `Amr (ra ), that the Prophet (saas ) said:
"Whoever is turned back from his objective by a bad omen has committed Shirk." They
asked: "And what is the expiation for that?" He (saas ) replied: "It is to say: "Oh, Allah
(swt )! There is no good except that which You bestow and there is no evil except that which
You bestow and there is none has the right to be worshipped but You."

Wa Aleikumus-Salaam.