You do. How often depends on how well you take care of it and where it is. I have a friend who has a tattoo on her hand, and she will have to get that retouched eventually. Because your hands are constantly exposed, the sun will fade it. Every tattoo artist is different, some offer touch ups for free, or even cheap. Where other tatt artists don't offer free or low cost touch ups for tatts that need frequent upkeep.
Whereas with a back tatt, if you put sunscreen on it and keep it out of a lot of direct sunlight, will last longer. There is no specific time for how often you retouch it. Just get it done when it starts to noticeably fade if you choose.
I suggest bringing your tatt idea into the tattoo artist and discuss with them these things. They can give you a better idea of what upkeep you will need and even give you some great tips for taking care of your tatt. They are better to ask since they can see what you want and where you want it.
When you first get them done or then onwards? Well I've only had a few touched up because the ink has come out in places but pretty much all of mine have stayed the same way. Most tattooists offer free touch ups.