Sometimes it is seen that the sadhakas are engaged in devotional service, and after prolonged sadhana they give it up because they lose their faith. Bhaktivinoda Thakura says that patience is required. To have patience means that a devotee must have firm faith that he will get Krishna’s mercy—either now, one year from now, a hundred years from now, or a hundred lifetimes from now. With this faith that someday he will get Krishna’s mercy, he continues to serve Him.
LFD Vol 3, Establishing Our relationship with Lord, Pg 65, last paragraph."\