It is impossible to pray forever. But when we are not praying, we are to have a prayer in our hearts. The Savior commanded the Nephites to pray until the time had come for them to stop. At that point, he commanded the multitude that they should cease to pray, and also his disciples. And he commanded them that they should not cease to pray in their hearts (3 Nephi 20:1).
Joseph F. Smith
"The prophets of the Book of Mormon have enjoined upon us that we should carry with us always the spirit of prayer in our hearts, morning, noon and night, and that we should pray for the blessings of the Lord upon our families, our farms, our flocks, our herds, our business, and everything that we possess in the world. Do not forget to pray. Don't suppose for a moment that you are as safe and secure in the favor of the Lord when you feel independent of Him as you will be if you feel your dependence upon Him all the day long." (Conference Report, Apr. 1915, p. 140)
Elaine L. Jack
"We are women with spiritual strength. We have within us the Spirit of the Lord to guide us on the path of righteousness. Daily, we choose between temptation and sanctification. We seek spiritual moments as we read the scriptures. We pray and carry that prayer in our hearts." (Church News, 10/02/93)