In Jesus' instructions about prayer, He taught His followers: "And when you pray, do not keep on babbling like pagans, for they think they will be heard because of their many words" (Matthew 6:7). These words of Jesus were aimed straight at the Pharisees. These individuals always made a public show of their prayers. They would typically pray in a public place--perhaps on a street corner--to impress people with their piety. They would pray conspicuously. They very much enjoyed being seen as they prayed.
The Pharisees also made their prayers excessively long, a practice picked up from the pagans, who engaged in endless repetition and incantation. An example of such endless babbling is found in 1 Kings 18:26: "They called on the name of Baal from morning till noon. 'O Baal, answer us!' they shouted." The belief was that endless repetition of specific requests endeared the petitioner to God, and hence God would be obligated to answer. Prayer was used by the Pharisees as a lengthy formula or technique to manipulate God into action.
So--the point of Jesus' instruction is not that we should necessarily utter short prayers to God (although short prayers are just fine if that is all you have time for or if that meets your particular need at the moment). The point of Jesus' instruction is that we should not engage in endless babbling, repeating the same request over and over again within the confines of a single prayer, as if that would force God's hand to answer. God answers prayer not because He can be moved to do so by endless babbling but rather because He desires to do so as your heavenly Father.
In support of the fact that Jesus is not forbidding long prayers is that Jesus Himself is portrayed as praying at length (Luke 6:12). He also repeated Himself in prayer on occasion (Matthew 26:44). He further instructed His disciples that "they should always pray and not give up" (Luke 18:1). Jesus' point was not that one should avoid long prayers, but one should avoid the Pharisaic misconception that prayers are effective precisely because they are long.
