I agree with the previous answers. "Giving back" should be a part of any growth plan. I view sharing a percentage of your knowledge and expertise as one method of giving back.

There were certainly people in my early career that simply gave - expecting nothing in return. They are the reason I am in the position I am today.

That said, if you "don't have time" to help others by answering questions, look at where you are spending your time and money. Your priorities will quickly become evident. Spend 10% of your time giving back and the rewards will exceed your expectations.