It is said that if you let your runner/french beans reach full maturity, the plant stops producing.
So I harvest my beans (even if only to throw/give them away) regularly to prolong cropping.
How thorough do I need to be? Is it sufficient to regularly pick most of the beans that are ready or do I have to ensure I catch every single bean pod?
This is a good question. I have to admit that, so far, I haven't been brave enough to allow any pods to reach full maturity. In my experience, however, it isn't necessary to harvest every single bean pod before it begins to ripen. I usually harvest mine every few days, for about six weeks, before the beans inside have begun to swell, and I've found that if I miss the odd one that is less visible, and it begins to ripen before I spot it, this doesn't switch off flowering; however, I only ever miss two or three at the most (not all on the same plant), and I imagine that if more were allowed to start ripening or reach full maturity, the plants would stop producing.
I have the opposite situation from Mancuniensis: I only harvest dry beans, so I never pick them before they are mature. I haven't studied them to find the exact moment when they stop flowering, but in my experience, flowering seems to slow down when the beans start to bulge inside the pods.