Generally 12 inch overhangs are common. In some instances with soft stone such as marble or something heavily veined the slab may need to be reinforced from underneath with steel rods. An attractive alternative to granite is quartz like Caesarstone or similar products. They are hard, durable and do not require sealing and resealing.

You best direct that question to your stone fabricator. As Urbanata mentioned, type of stone, its veining and thickness impact the decision.