http://www.innas.com/Assets/files/Hydrid%20brochure.pdf
I've posted this link several times before. It shows a comparison and explains the advantages, even allowing for a more efficient manual transmission.
How long the ice needs to run to fully recharge the accumulator depends on the load the vehicle is encountering Neil. Speed and grade both affect total load, while braking offers an opportunity to replensih the accumulator without fuel consumption or engine operation. The linked article mentioned 11% total engine run time in the vehcle during the run through the euro cycle. The engines efficiency was doubled compared to a normally operated manual transmission.
regards
Mech