There is no formula for it. You sometimes need to use mathematical theory to prove upper and lower bounds on the order of an operation, and you may need to do some tricky summations to provide an average order estimate.
In very simple algorithms you can work it out easily, but in algorithms as sophisticated as PSO and GA, you are going to have difficulty. In fact, you will find that for GA at least, the complexity order depends upon the function being minimized (I do not know PSO well enough to know what the stopping criteria are for it.)
