Toddlers and preschoolers have short attention spans and sitting at the meal table can prove a challenge for them. But, if they are hungry, they tend to eat most of their meal in the first 5-10 minutes.
Young children are learning your expectations, and you want your expectations to be realistic. A toddler should be able to sit for 5-10 minutes at mealtime and less for a snack. A preschooler can sit longer: for 10-15 minutes for a meal. If this is a stretch, train her. Use a timer to indicate when she can be done eating and excused from the table. Making your toddler or preschooler sit for extended periods of time will likely lead to poor eating and behavior. As your child gets older, her ability to sit for longer periods of time will emerge.