What is the length of the journey that makes it permissible for one to offer a shortened Salat (prayer)?

The Prophet (peace and blessings of Allah be upon him) has called the distance of one day and one night a journey. And Abdullah ibn Umar and Abdullah ibn Abbas (may Allah be pleased with them) would shorten the prayer and break the fast at a distance of four barid (approximately forty-eight miles). There are sixteen farsakh in four barid (and one farsakh is three miles).

Narrated Ibn `Umar: The Prophet said, "A woman should not travel for more than three days except with a Dhi-Mahram (i.e. a male with whom she cannot marry at all, e.g. her brother, father, grandfather, etc.) or her own husband.)"

Narrated Ibn `Umar: The Prophet said, "A woman should not travel for more than three days except with a Dhi-Mahram."

Narrated Abu Huraira: The Prophet (p.b.u.h) said, "It is not permissible for a woman who believes in Allah and the Last Day to travel for one day and night except with a Mahram."