Here's some interesting GPS tracking data:
The electric motor assistance shuts-down somewhere between 20-25 km/h, and furthermore I only use it for going uphill (the Brompton was really heavy thanks to a decent shopping load)
I'd estimate that on the steepest hills I only contribute 60% of human effort. The rest of the time I am probably providing between 70% and 80% of the effort required (anything below the 20-25 km/h speed
At any rate, I have now learnt how to properly use the pedelec mode (choice of gearing, etc. due to speed-only sensor, no torque sensing), and I am beginning to enjoy it !