In today's Gospel reading from Matthew 8, the centurion asked Jesus to heal his paralyzed servant. When the Lord was ready to go, the centurion declared that he's not worthy to receive him and only asked him to say the word and the servant would be healed. The Lord was impressed by his faith and the request was granted.
My thoughts are that if we have faith, we shouldn't ever wonder or worry if the Lord is going to answer our prayer or not. To persist and ask again is fine, but to lose peace afterwards, no. Faith without trust is empty.