It will really depend on the size of your pork chunks and the heat of the grill. But if you have cut the pork into 2-inch by 2-inch chunks and are using very high heat, you'll likely need only a few minutes per side. If using high-quality pork, you needn't worry about making sure the skewers are well done (medium to medium-well is fine). They should be fully cooked through but not overcooked (or they'll be dry).