Matthew 6:7-8

When you pray, do not babble repetitiously like the Gentiles, because they think that by their many words they will be heard. Do not be like them, for your Father knows what you need before you ask him.

This is really more a depth question than a length question. Pride can be at work even here in the heart of the person praying - which is a danger whether your prayer is very long, or very short. Approach God as who He is: our loving heavenly Father. He knows you perfectly, knows what you need, and is able to answer! Trust Him with your heart, because He has proven faithful and His love is known as "unfailing". There is no need to impress God with many words, simply reveal your heart to Him in prayer.