This can alter from season to season!

Usually the onions indicate this themselves in so far as the tips of the leaves turn yellow-ish and the leaves flop to the ground.

I do not believe in bending them over manually simply because if the leaves are erect the plant/s are still growing.

This link might help you as it gives a general appreciation on how to grow, lift and store onions;

http://www.thegardenersalmanac.co.uk/Data/Onion/Onion.htm