Adam Clarke on Luke 18:1

Men ought always to pray - Therefore the plain meaning and moral of the parable are evident; viz. that as afflictions and desolations were coming on the land, and they should have need of much patience and continual fortitude, and the constant influence and protection of the Almighty, therefore they should be instant in prayer. It states, farther, that men should never cease praying for that the necessity of which God has given them to feel, till they receive a full answer to their prayers. No other meaning need be searched for in this parable: St. Luke, who perfectly knew his Master's meaning, has explained it as above.