According to Genesis, Noah waited seven days after entering the Ark before the Flood began. Genesis 7:10 says that the Flood waters came after this seven-day interval.
The seven-day wait is separate from the forty days and forty nights of rain. Genesis 7:4 announces the coming rain, while Genesis 7:12 later describes rain falling for forty days and forty nights. These are different time markers in the narrative.
Genesis also gives other durations connected with the Flood, including 150 days during which the waters prevailed and the eventual drying of the ground. Because several numbers appear close together, summaries often merge them into one generalized period.
The seven days specifically describe the interval between Noah’s entry with his family and the start of the waters. They do not describe the duration of the rain.