I switch off at traffic lights if I know I am in for a bit of a wait. There are certain sets of lights that you know, if they change to red just as you approach them, will be red for a spell. These are the times I switch off. They reckon that you need to be switched off for more than 7 seconds to see any benefit.
I did an Energy Trust free driving course and the instructor told me, after out first circuit, to read the average mpg then leave the engine idling.We talked for two minutes then he said "What's it reading now?". It had dropped considerably. He told me always to switch off if stopped for any length of time. Now, I even switch off when I drop my wife at her work. By the time she gathers her stuff, gets out, walks off, then turns and gives me a wave, 15 or more seconds have passed.
Even with the number of starts and stops I do in a day I come nowhere near that of a doctor doing house calls, or a health visitor visiting patients. The manufacturers have built in a huge safety margin regarding wear and tear on the starting system. Cars with engine stop/start are fitted with AGM batteries, but that is to cope with a far greater stop/start cycle than I am looking at.
The guys on here that use their Jazz's for driving lessons will do 10 times the stops and starts in a day than I ever do. I would imagine their starter motors still last the lifetime of their cars (?)