That depends on many factors, such as your child’s usual activity level, the learning site and your own expectations.
In general, little kids can’t sit still or focus on one topic for long. They learn well through movement. To reach them, have short lessons and vary the method of learning (for example, moving ABC tiles, printing letters, playing Mother, May I? and walking through the house finding objects that begin with a certain letter). If the child thinks he is playing a game and can move around, he will hang in there much longer.
If you have set up a table where you want to work with your child, evaluate the setting from the child’s vantage. Does the chair allow her short legs to touch the ground? Or if the chair is higher, is there a shelf for her to rest her legs? (You can stack some books or a cardboard box under her feet.) Does the chair support the child’s back? (You can tie a pillow to the chair to fill the space if the chair seat is too deep.) Is the table low enough (waist-high or just a bit higher) so that her arms can relax while she works? All these accommodations to a child’s small size encourage a child to sit longer.
Some children prefer to stand while they work. They can stand straight, slouch, spread their legs or lean—quite a bit of movement all while staying in one place and paying attention. Some kids prefer to kneel or to squat on a chair or to stand on a chair and bend at the waist. The child might look absurd, but if he is paying attention, who cares?
Maybe the real question is whether you are comfortable teaching a squirming child? If you are not, let the child know up front that the lesson will last only a few minutes. Use a timer so the child knows that when the timer rings, he can move around.
If another adult is watching, you might feel pressure to make your child sit still. But is this in your child’s best interests? If the child feels pressured to conform to some standard that he is not ready for, he will not learn as well as if he is relaxed.