An outdoor fish tank? I have never heard of such a thing.
I would imagine you would want to clean it more often as it is more likely to develop algae and bacteria being outdoors. So whatever is recommended for the size of tank you have, do it twice as often as that.
This is the first time I have heard of an outdoor fish tank, but it does have me intrigued. I guess with it being outside and possibly in different temperatures and also possibly in sunlight quite often, you would definitely clean it more often than you would a tank that is in door. I myself tend to clean my indoor tank once a month fully with water top ups throughout the month. I would advise cleaning yours maybe at least once a week with it being outdoors and more things like algae would build up being outside.