The route has to stay within 120 mins IN STILL AIR AND ISA at the given single-engine ETOPS speed approved by the regulator. Just simple circles on the chart! If you are studying for an exam question you will be given this figure.
138 mins after 3 months of 120 mins operations as far as I know.
Try Google for 'ETOPS' or search PPrune for more. AB have a good publication.