Such order may specify the time during which the relatives aforesaid shall maintain such poor person, or during which any of the said sums so directed by the court shall be paid, or it may be indefinite, or until the further order of the court; and the said court may from time to time vary such order, whenever circumstances shall require it, on the application either of any relative affected thereby, or of the county department of social welfare, or of the bureau of social aid of the county department of social welfare.