Rough estimate of running time from Big O

Question

If the time complexity of my program is,say O(n^2),How do I express running time in terms of seconds for a large value of

Answer 1

You need to roughly know how much one of your base tasks takes in order to have an estimation of the running task for different Algorithms.

As an example, let's imagine your base task is

void func(){sleep(1)};

now you know that a O(1) complexity algorithm will yield to just one call to func(), which will take 1s.

Looking at other examples:

O(1) -> 1 * 1s
O(N) -> N * 1s
O(N2) -> (N^2) * 1s

Without having a rough estimation of your task's execution time, it is impossible to give a precise answer.