When we ask God for something (and what we're asking is God-honoring), then we need to persevere and continue knocking, no matter how long it takes! He wants us to ask Him for things according to His will, and to keep asking until He answers. But when God shuts a door, He shuts it with a bang -- and the best way to hear His voice in our requests is to let Him in when He knocks at the door of our own hearts.