During the summer months or warmer months you should water your gardenia plants 2 to 3 times per week or when the soil starts to become dry.
When the soil becomes dry the gardenia plants can start to die off so make sure to keep the soil that the gardenia plants are planted in moist.
So feel the soil and if it's getting dry it needs some water.
My gardenia plants need watering at least 3 times per week in the summer but in the winter months watering the gardenia plants every other week is usually enough water to keep the gardenia plants alive and thriving.