There are many factors involved in determining a safe speed for a left turn. Weather and road conditions. Type of vehicle.. Motorcycle? small car? suv? pick-up truck? Different vehicles handle differently.
In most left turn situations, I find myself at a complete stop, waiting for lights or cross traffic before making the turn, which makes the speed irrevelant.
However, I would say in an average car, on dry pavement, with a clear approach, I would tend to keep it at or below 20 mph for the turn.