A decade. And it happened in two spikes with the the first being followed by a retreat in oil prices limiting the effect (oh, ok, that was just an aberration, no need to change). The latter part of that period did see significant gains in vehicle fuel efficiency, but the two decade long slump in fuel prices that followed quickly put an end to that, at least in the US. In Europe the policy of keeping gas prices high helped preserve the trend, but that's been partially counterbalanced by the simultaneous completion of the switch over to universal car ownership and the shift to a more American car based social geography (shopping outside city centers and suburbanization)