Not all floods are created the same way - it may take days for flood waters to rise or just a matter of minutes.
Floods are often times a gradual but persistent process. The key for a flood is that the ground needs to become saturated so it can't absorb any more water or water levels for streams, creeks, and rivers become too high. When this happens water will rise, and when this happens, flooding occurs. The ground can become saturated due to persistent rain, snow melt, or previous rain storms passing through. Regardless of the cause water needs to go somewhere, so when the ground or rivers can't take any more water it will flood.
Flash floods can be more dangerous because they happen quickly. This is why they are called "flash" floods. These occur when too much rain falls in a short amount of time that the ground can't absorb it or rivers take it more water than it can move downstream. Summertime thunderstorms can lead to flash flooding due to downpours and heavy rain in a short amount of time.