Matthew 6:6-7 But thou, when thou prayest, enter into thine inner chamber, and having shut thy door, pray to thy Father who is in secret, and thy Father who seeth in secret shall recompense thee. And in praying use not vain repetitions, as the Gentiles do: for they think that they shall be heard for their much speaking.

Repeating the same words over and over like a magic incantation is no way to ensure that God will hear your prayer. It's not wrong to come to God many times with the same requests—Jesus encourages persistent prayer. But he condemns the shallow repetition of words that are not offered with a sincere heart.
Not that all long prayers are forbidden; Christ prayed all night, Lu. 6:12. Solomon’s was a long prayer. There is sometimes need of long prayers when our errands and our affections are extraordinary; but merely to prolong the prayer, as if it would make it more pleasing or more prevailing with God, is that which is here condemned. This caution is explained by that of Solomon (Eccl. 5:2), Let thy words be few, considerate and well weighed; take with you words (Hos. 14:2), choose out words (Job 9:14), and do not say every thing that comes uppermost.
We can never pray too much if our prayers are honest and sincere. Before you start to pray, make sure you mean what you say.
Luke 11:1  'Lord, teach us to pray...'

(Tyndale)