If they're a good manager who has been consistent in having your back and is supportive of growth opportunities, then I would give them a heads up (maybe not 6 months in advance if you need the income and can't guarantee they won't let you go). It really just depends on how solid your working relationship is, your employer's staffing needs, and your comfortability with the risk of being let go with how many months in advance, you decide to tell them.