The same way you do for any other operation:
"You add up how many machine instructions it will execute as a function of the size of its input, and then simplify the expression to the largest (when N is very large) term and can include any simplifying constant factor."
Source:
How to find time complexity of an algorithm - Stack Overflow[
^] - and it's about the most succinct definition I've seen!
But basically, it means exactly what ppolymorphe said: "The answer depend of actual code."
More detailed explanation as to how to work it out:
Complexity and Big-O Notation[
^]
This isn't something we can do for you.