Your child is now able to sustain a longer attention span. But there is no hard and fast rule on how much time the attention would be retained by your child. It differs from child to child as well as the situation. So as a parent you can increase their attention by paying more attention to what you want them to do and how to make the activity interesting to them. You can be more creative and that helps them to draw their attention.
What you can do
One more way is to lessen their distractions. Check if they are hungry or sick or have any other problems, if they are not able to show interest/attention for a certain period of time even after your repeated trying. Try giving a light snack before starting with their homework – homework is one activity most 7-8 year olds do not want to do, and show less attention span, while they can sit for a longer period of time while playing blocks, puzzles, etc.
Give them time to recoup after coming back from school to start their homework and other school activities. The other distractions could be a younger or older sibling or a playmate. Try to provide utmost attention to the kid in question and separate them from their siblings and playmates so they have the feeling that your complete attention is on them, which helps them in paying attention for a longer period of time.