Most studies on start-stop technology show real-world fuel use reductions of 5 to 10 percent, depending on driving patterns. One study found that start-stop begins to save fuel when the engine is off for as little as seven seconds during an idling period.
I completely believe this and also absolutely hate the feature because it is a net negative outside of dense cities with a lot of stop and go traffic. Plus my particular model loves to restart after anywhere between 5 to 20 seconds, seemingly triggered by people talking since it even happens a while after putting the vehicle in park. My wife's model is better, it generally stays off when not moving.
But in both cases, it introduces problems with driving on the roads we most commonly use. Stop/start absolutely sucks when it kicks in at roundabouts or when stopping to let oncoming traffic go before turning left. Yes, you can let off the brake a little to get it going again but then it just wasted gas by not being off long enough for a benefit. Don't even get me started on stop signs, where it is off for far less than a second. While there is an option to turn it off in both vehicles, it then displays a prominent bright yellow warning looking light which is distracting and it has to be turned off every single time the vehicle is started.
I disabled mine after a year of being annoyed, and now I get 10% better gas mileage because my driving patterns don't involve stopping for more than the 7 or seconds often enough to break even. I do manually shut it off at drive throughs, which saves more than the automatic process since it doesn't start up until I'm ready to go.
Honestly I would love for easier control over stopping it temporarily myself than having an automatic system that assumes stop signs take more than 7 seconds.