The velocity of a planet in a circular orbit is just the circumference of the orbit, 2 x pi x radius, divided by the time it takes to go once around in its 'year'. So, that means that since
2 x Pi x Radius
Velocity = -------------------
Period
that
2 x Pi x Radius
Period = -------------------
Velocity
Just make certain you use the same units in velocity as you do in
the radius of the orbit, example, Radius in kilometers, Velocity in
kilometers/second gives period in seconds. For planets in highly elliptical
orbits, the formula is more complex, but is still useful to get the order of
magnitude right!