Calculating speed

Is it simply 125 x 60 x 60 = 450000 MPH?

that doesn't make any sense. You don't measure speed in time.

You measure speed in distance/time.

What I think you mean is that the eye sees everything that is visible in a single spot for at least 1/125th of a second.

So the speed, something needs to travel at, to become invisible, would be dependable on the size of the object, relative to the object the eye is fixed on. If your eyes are fixed on the object it cannot turn invisible at sublight speed.

Edit: Also wrong forum.

Thank you for your comments, daAresinger.  We will assume a distance of 1 mile for this calculation.  My apology for not making that clear.

As this is my first post on this site, could you please advise a 'proper' forum?

i would suggedt gerneral questions.
you can simply edit yourpost and move it over there