Question about changing MH ballast

[Casey8]

I am wondering how often do you need to change your MH ballast, or do you use it forever ? I have a ballast that I have been using for over 3 years now, do I need to change it like we change bulbs, maybe not as often as bulbs. How can you tell if it is too old and when to get a new one ? Boxing day is coming soon, I think I'd better ask now so I can add it on my list.