Praise be to Allah.
For breast-feeding to have the effect of making a child a relative (mahram), two conditions must be met:
(1) The number of breast-feedings should be five or more, because of the hadith of ‘Aa’ishah (may Allah be pleased with her), who said: "When the Qur’an was first revealed, the number of breast-feedings that would make a child a relative (mahram) was ten, then this was abrogated and replaced with the number of five which is well-known." (Reported by Muslim, no. 1452).
(2) This should happen within the first two years of the child’s life, because of the hadith: "Harmalah ibn Yahya told us that ‘Abdullah ibn Wahb told us that Ibn Lahimah informed us from Abu al-Aswad from ‘Urwah from ‘Abdullah ibn al-Zubayr that the Messenger of Allah (Peace & Blessings of Allah be Upon Him) said: "There is no breast-feeding except what fills the stomach to bursting point." (Reported by Ibn Maajah, no. 1946; see also Saheeh al-Jaami‘, no. 7495). Al-Bukhaari, may Allah have mercy on him, included a chapter in his Saheeh entitled: "The chapter of the one who says there is no breast-feeding after two years because of the words of Allah ‘. . . two whole years, (that is) for those (parents) who desire to complete the term of suckling . . .’ [al-Baqarah 2:233]."
The definition of rad‘ah (one breast-feed) is that the child should take the breast and drink milk from it, then leave it of his own accord in order to breathe or to change position, and so on.
If this is the case, then the rulings concerning breast-feeding will apply, i.e. the child will be considered a relative and marriage will be forbidden, etc.
In the case of doubt concerning the number of breast-feedings, Ibn Qudaamah, may Allah have mercy on him, said: "If there is some doubt concerning whether breast-feeding took place, or concerning the number of feeds and whether it was enough to make the child a relative (mahram) or not, then there is no proof that the child must be considered a relative, because the general rule of life is that a child does not breast-feed from anyone except his own mother, and one cannot dismiss certainty in favour of doubt. (al-Mughni 11/312).
Therefore it is permissible to marry if there is no proof of breast-feeding that would make the individual concerned a relative.
I do not want to omit reminding the one who asked this question that we are obliged to adhere to the Sharee‘ah no matter where it leads us, and our desires and emotions should not prevent us from following the truth. The Muslim must remain chaste and avoid all kinds of "love relationships." He must strive to protect himself by marrying in a proper fashion according to Islamic sharee‘ah.