Mango trees still need watering in Winter here in SoCal but much less frequently. If your tree is fully established, it may only need watering every 3-4 weeks but it depends on many factors including rainfall, size of your tree and state of growth/production it is in.
If your tree is established and large enough to hold Fruit to maturity you can continue to fertilize with Potassium a little Phosphorus some Calcium and micronutrients to help sweeten and size up the fruit.
Simon