You can run it a thousand times and get an average of 5s, but you will never be sure the next invocation will take 5s. You can bet it probably will do, and for most purposes is totally fine, but you will never be sure.

Never exactly tho, and probably not even precisely. What if the computer is also doing other stuff. Or the process relies on I/O and network, etc.

That is how long it took, not how long it will take.