The FAA addressed this many years ago (yes, in the piston airliner years, carried over to jet ops) and it was called 'island reserve', and as mentioned above, 2 hours normal fuel reserves at the normal cruise speed.
Used mostly in the Pacific, at least from my personal experience...on pistons and jets.
The wx forecast and runway layout needs to be considered with absolute caution, least you be caught out short of gravy.
PanAmerican used this procedure with many early B707 ops to some Pacific island destinations, as some of the straight pipe models gulped fuel like a big Hoover, with a lower than optimum assigned altitude.