If you follow the routine maintenance most aquariums provide, you should be able to clean your outdoor fish tank once a week. If the aquarium is in a highly visible area of the house, you may want to schedule cleanings twice a week.
Every fish tank is going to need regular cleanings. The fish, the food, the plants and even light will all affect what happens on the inside of the tank. You want to keep the fish healthy so you'll do the same types of things you'll do for your inside tanks.
Yes, without a doubt no matter where your fish tank is, it would need to always be cleaned out regularly to prevent any issues with the water that could affect your fish. I would always say that an outdoor tank should be cleaned out more a lot more regularly than an indoor tank, say once a week would be efficient enough to keep things clean and running as they should.