There is no such rule like that. On the contrary people are advised to initially work temporarily as volunteers and see for themselves if they can get in sync with the Ashram life. There are others who never join the Ashram but work as volunteers going back and forth between the Ashram and their home. There are others, though few, who join the Ashram but leave and go away for different reasons. Spiritual evolution is not a simple straight-jacketed formula of life but a many-sided wide and plastic movement. There are therefore no fixed and absolute rules or binding formulas. To each his own unique journey.
Affectionately,
Alok Da