That's not true at all, there are a bunch of different factors that go into when the start/stop kicks in. For one the capacitors that allow it to restart must have a charge, which doesn't typically take long, but I'm sure that depends on the driving type/style. Also if you have the AC cranked or heat cranked it won't kick on.
For me, I usually stop normally, and when the car comes to a complete stop, if I've been driving for a little bit it'll stop. I'll typically press down harder on the brake to get it to start the motor back on before I try pulling out of an intersection.
I have about 3k miles on my car now, almost an hour of "Stop" time, it's saved a whole ... 0.39 gallons of gas. So not worth the savings in my mind, really wish someone would come up with a permanent delete/disable.