Trying to find Worst Case complexity
The computational complexity depends on what you want to measure. If you you're doing this computation on a common computer with floating point arithmetic, then you can estimate it mi $O(1)$, because every floating point operation (addition, multiplication and square root) is in $O(1)$.

If you use fixed point arithmetic for $n$-bit numbers, then addition can be done in $O(n)$, multiplication in $O(n^2)$, and square root in $O(n^2)$ steps (Note: For multiplication and addition are existing better bounds). Putting this together, you will add two $O(n)$-bit numbers and take the square root of the result which gives $O(n^2)$ steps.

As you can see, the runtime depends on the arithmetic used, and arithmetic determines what $n$ means. If you use $n$-bit complex number in a $n$ dimensional vector space (where $x^2$ denotes the scalar product), you probably will get other estimates.