The Templars are an appelate body, they created a requirement that you be a Master Mason in good standing in order to belong to them. Therefore "a certain period of time" is very different in different areas. That particular appelate body stresses Christianity. There is not specific time limit, only that you be a Master Mason, and the reason for that is because only then are you charged yearly dues and can be recorded as a mason in good standing based on that. This stresses that it is the Blue Lodge (only 3 degrees) that constitute Freemasonry, if you give up that all appelate bodies will reject your membership as you are no longer a Freemason.