Try this:
import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BOARD)
GPIO.setup(7, GPIO.OUT)
while True:
GPIO.output(7, True)
time.sleep(10)
GPIO.output(7, False)
time.sleep(5)
It should loop the on/off sequence, causing the light to turn on for 10s, then turn off for 5s, and repeat.