If a righteous prayer by parents for their children is answered, does its effect last a lifetime or is it limited to a single purpose or a one-time occurrence?

The response to the supplication of parents and other supplications rests with Allah Almighty. He may answer the supplication and grant success to the servant, or He may answer in one matter but not another, or He may store it as a reward in the Hereafter, or avert some evil through it. Allah knows best what is beneficial, and there is no repeller of His will. The servant must supplicate with sincerity. Parents should supplicate for their children with devotion and entrust the matter of the response to Allah Almighty, for He is the Most Merciful to His servants.