A typical child can never focus on a sole task and that is because of their inherently less attention span. Children will do a dozen different things within a span of an hour. This trait is quite natural. A child finds the world around him or her extremely interesting and wants to experience anything new as soon as it is shown.
As a parent, it is your duty to teach your child how to concentrate. Small children should be able to finish tasks like completing a puzzle or making a block tower. Such as with martial arts, children will expand their ability to focus on a goal. Of course, not every child increases this ability at the same rate. You will notice a growth in this area of development; for instance, they will patiently stand in line for movie tickets as opposed to becoming impatient the more they have to wait. In one sentence, your child will grow up to be a complete individual!
Since we are older, we can shut off distractions and have gotten better at getting tasks done. We can ignore the sound of traffic outside the house, but the same noise can disrupt the focus of the child. For young children, ambient sounds are louder and they are more sensitive to smells; if injured, for them it pains more. As a parent, it should be your job to minimize such distractions or eventualities.
Keep your child engaged
Do not think about how long a time your child does a specific activity. Take into account the interest level instead. If you see your child abandoning the blocks after placing only two of them, make the little one do one step more. There will be an increase in the focus and attention span. The child will also learn how to manage frustration. Exposing your child to valuable life skills earlier in life will help discipline, focus and help them learn easier as they grow up.