I seem to recall reading somewhere about a legal requirement to have a certain amount of reserve fuel,
ANO 2005 simply requires that
sufficient fuel, oil and engine coolant (if required) are carried for the intended flight, and that a safe margin has been allowed for contingencies,
ICAO Annex 6 (applicable to international GA) says
4.8 Fuel and oil supply
4.8.1 A flight shall not be commenced unless, taking into
account both the meteorological conditions and any delays that
are expected in flight, the aeroplane carries sufficient fuel and
oil to ensure that it can safely complete the flight, and, as
applicable, the following special provisions are complied with:
4.8.1.1 Flight in accordance with the instrument flight
rules. At least sufficient fuel and oil shall be carried to allow
the aeroplane:
a) when, in accordance with the exception contained in
4.6.2.2, a destination alternate aerodrome is not
required, to fly to the aerodrome to which the flight is
planned and thereafter for a period of 45 minutes; or
b) when a destination alternate aerodrome is required, to
fly to the aerodrome to which the flight is planned, thence
to an alternate aerodrome, and thereafter for a period of
45 minutes.
Note.— Nothing in 4.8 precludes amendment of a flight
plan in flight in order to re-plan the flight to another
aerodrome, provided that the requirements of 4.8 can be
complied with from the point where the flight is re-planned.
Then the FARs (applicable in the US and to N-reg) have the explicit 30/45 min requirements:
Sec. 91.151 Fuel requirements for flight in VFR conditions.
(a) No person may begin a flight in an airplane under VFR conditions
unless (considering wind and forecast weather conditions) there is
enough fuel to fly to the first point of intended landing and, assuming
normal cruising speed--
(1) During the day, to fly after that for at least 30 minutes; or
(2) At night, to fly after that for at least 45 minutes.
Sec. 91.167 Fuel requirements for flight in IFR conditions.
(a) No person may operate a civil aircraft in IFR conditions unless
it carries enough fuel (considering weather reports and forecasts and
weather conditions) to--
(1) Complete the flight to the first airport of intended landing;
(2) Except as provided in paragraph (b) of this section, fly from
that airport to the alternate airport; and
(3) Fly after that for 45 minutes at normal cruising speed or, ...
[exceptions follow]