With 10 guppies in 180 litres, even if the tank and media were brand new, ammonia would not build up very fast. Test once a day 
Is this a liquid reagent test? Just to warn you of a quirk with the ammonia tester. Fluorescent lights can make the liquid look greener than it really is. Ideally test in daylight, or use a halogen/LED light.
Is this a liquid reagent test? Just to warn you of a quirk with the ammonia tester. Fluorescent lights can make the liquid look greener than it really is. Ideally test in daylight, or use a halogen/LED light.