I have often been told that “pray without ceasing” (1 Thess. 5:17) and “praying at all times” (Ephesians 6:18) means “pray a lot”. Apparently Muller didn’t hear that sermon:
The letter [Pierson references a letter written by Muller] makes plain that much sweetness is mixed in the cup of suffering, and that our privileges are not properly prized until for a time we are deprived of them. He particularly mentions how secret prayer, even when reading, conversation, or prayer with others was a burden, always brought relief to his head. Converse with the Father was an indispensable source of refreshment and blessing at all times. As J. Hudson Taylor says, Satan, the Hinderer, may build a barrier about us, but he can never roof us in, so that we cannot look up.
Arthur Tappan Pierson. “George Müller of Bristol.”