As a general rule, lack of support should be avoided in your design. If it cannot, for example for a breakfast bar an overhang or “cantilever” of 200-400mm is fine. For unsupported spans 600-900mm is acceptable but this depends on thickness and fabrication method. To provide additional strengthening to a span, reinforcement mesh can be used at the base of the concrete to counteract a sag.
Take a look at a video we took testing our worktop premix kit concrete.