According to the Vedas or Gita does the Antim sanskar last 3 days, 13 days or one day? Is there any written instructions in any veda or the Gita?
Both the Veda and Gita talk about SPIRITUALITY in most of the places, Puranas talk about rituals.
In Ramayana it was mentioned that Bharata got himself purified on 12th Day and collected the bones on the 13th day.
ततः दश अहे अतिगते कृत शौचो नृप आत्मजः | द्वादशे अहनि सम्प्राप्ते श्राद्ध कर्माणि अकारयत् || २-७७-१
After having passed the tenth day of the death of Dasaratha, Bharata having himself purified, performed Shraddha* rites on the arrival of the twelfth day.
ततः प्रभात समये दिवसे अथ त्रयोदशे || २-७७-४ विललाप महा बाहुर् भरतः शोक मूर्चितः | शब्द अपिहित कण्ठः च शोधन अर्थम् उपागतः || २-७७-५ चिता मूले पितुर् वाक्यम् इदम् आह सुदुह्खितः |
Then, at the time of dawn on the thirteenth day, the mighty armed Bharata who came for clearing up bones and ashes, wept in grief, his throat being choked of sound (because of crying) and spoke in great distress the following words, having reached the foot of his father's funeral pile (to collect the bones and ashes for their immersion in Sarayu River).