A. At the end of Dwaparayuga
Q. By Whom?
A. By VedaVyasa
A. Considering shortened strength and life-time (incapability to learn whole Veda) in Kaliyuga
A. According to Mantras used in Yajna by Ritvij
According to Srimad Bhagavatam 12.6:
Vedas are manifested from OMkara:
One perceives this [eternal, itself imperceptibly manifesting] sound, outside the physical sense of hearing and power of vision. The complete of the Vedic sound one employs is an elaboration of this omkâra that from the soul manifests itself in the ether. It constitutes the direct expression of the self-generating Absolute Truth and Supersoul, it constitutes the eternal seed of the Vedas and constitutes the secret of all mantras [see also 7.15: 31, 9.14: 48, 11.14: 34-35, 11.21: 36-40].
Vedas inherently constitutes Rik, Yajur and Sama hymns (even before the division):
Which inherently constitutes Rik, Sama and Yajur (even before the division of Vedas)
Oh eminence of Bhrigu, the three sounds of the alphabet beginning with A that came into being [the sounds A, U and M], are fundamental to [all] the threefold forms of material existence: the modes [the gunas], the names [of the three Vedas], the destinations [the three types of lokas] and the states of consciousness [avasthâtraya].
Brahma manifested four Vedas from his four faces:
For the purpose of giving instruction on the four sacrifices [see ritvik], the almighty one created with all these sounds from his four faces the four Vedas, along with his omkâra and vyâhriti invocations [of the names of the seven lokas].
Brahma taught Vedas to his son as one (complete set - without division):
He taught them [as a complete set] to his sons who were the great rishis among the brahmins most expert in the art of Vedic recitation, and they in their turn imparted them as instructors of dharma [âcâryas] to their sons.
Touching the question:
These headed down in in every Yugas upto Dwapara and divided at the end of Dwaparayuga:
ते परम्परया प्राप्तास्तत्तच्छिष्यैर्धृतव्रतैः ।
चतुर्युगेष्वथ व्यस्ता द्वापरादौ महर्षिभिः ॥ ४६॥
Throughout the four yugas generation after generation, one after the other disciple fixed in his vows received them [these Vedas] by disciplic succession [paramparâ]. They were divided among the prominent sages at the end of Dvâpara-yuga.
The reason for division is incapability and short life-period of man near to Kaliyuga: (as I mentioned in this answer)
The brahmin sages, inspired by the Infallible Lord situated in their hearts, came to that dividing among each other of the Vedas when they saw that under the influence of time the intelligence [of the people] diminished, the life span shortened and the strength weakened [see also 1.4: 16-18].
Observing that under the influence of kâla [the people became] lesser intelligent and short-lived and that their strength was diminished, divided the chief sages, inspired by the Infallible Lord situated in their hearts, the Vedas [see also 1.4: 16-18].
The one who has accomplished the task of division (in present Manavantara) is none other than Vyasa:
Oh brahmin, in this period [of Manu], Brahmâ and S'iva and other rulers over the worlds requested the Supreme Lord, the Protector of the Universe, to protect the principles of religion. Oh most fortunate soul, the Lord [in the form of Krishna Dvaipâyana Vyâsa], by Parâs'ara begotten in the womb of Satyavatî, then descended as a partial expansion of His plenary portion [Sankarshana] and divided the Veda in four.
How Vyasa divided Vedas (as I discussed in this answer) is explained in further verse:
Like someone sorting out gems, he separated the group of mantras in four categories of collections [Samhitâs]: the Rig, Atharva, Yajur and Sâma Veda [see Vedas].