Each woman is different, but a single hot flash can last between 1 and 15 minutes. Throughout the day, you might have anywhere between no hot flashes to several hourly - so there's a wide range of what is considered "normal."
The same is true for how long you can expect to be dealing with them. On average, women will have hot flashes for a period of 10 years, but many women experience them for decades!
Fun fact: hormone replacement therapy (HRT) is scientifically proven to be the single most effective way to decrease severity and prevent hot flashes.