The minimum duration of a journey which exempts the believer from fasting
is defined by Baha'u'llah (Q and A 22 and 75). The details of this
provision are summarized in the Synopsis and Codification, section
IV.B.5.a.i.-v.
Shoghi Effendi has clarified that while travellers are exempt from
fasting, they are free to fast if they so wish. He also indicated that the
exemption applies during the whole period of one's travel, not just the
hours one is in a train or car, etc.
31. The traveller, the ailing, those who are with child or giving suck,
are not bound by the Fast; they have been exempted by God as a token of
His grace. #16
Exemption from fasting is granted to those who are ill or of advanced age
(see note 14), women in their courses (see note 20), travellers (see note
30) and to women who are pregnant and those who are nursing. This
exemption is also extended to people who are engaged in heavy labour, who,
at the same time, are advised "to show respect to the law of God and for
the exalted station of the Fast" by eating "with frugality and in private"
(Q and A 76). Shoghi Effendi has indicated that the types of work which
would exempt people from the Fast will be defined by the Universal House
of Justice.