It depends on the species. Once Sulcatas hit about 6", they are pretty much bullet proof. My oldest two are about 4" now and I still mist several times a day.
I am sure others will disagree with me but it is my opinion that for as long as your tortoise is growing (so often their entire lives depending on the kind of tortoise you keep) they should have access to high humidity. I keep african tortoises; and in a natural environment their food is most abundant during the rainy season, meaning that they eat and grow the most while being rained on and kept very humid. All of the research I have done indicates that pyramiding is the result of dry conditions during periods of growth, so if you have a species of tortoise who grows for a loooong time then I would advise that you always maintain an area of high humidity for him/her. With my adult sulcatas I will "mist" them (spray them with hose or run sprinkler longer hours) anytime I notice the white rings of new growth forming around a scute. Misting and humidity may be especially important in a rapidly growing young tortoise but that doesn't mean it is not important as your tortoise ages.
I mist my male sulcata on hot summer days but I don't do anything extra to boost humidity since it will have no bearing on shell growth. My little one's have a repti fogger in there outside enclosure for extra humidity....
From most people I have talked to, it seems that once a sulcata is 6-8" the pattern for shell growth is pretty well set. I don't think you could take a smooth 8" sulcata and do anything to make him start pyramiding. This is also the smallest size that people in the natural habitat report seeing them above ground and out in the open. This is also the size that a lot of previously pyramiding youngsters start to smooth out on their own with no environmental changes. I don't think there is any harm in keeping them humid and hydrated longer than that, but this is about the size that I start to "relax" a bit with the "wet" routine.
If you look at an individual scute on any tortoise, you will see a center portion surrounded by concentric rings. This center portion, or shield, is what they are hatched with and all the new growth can been seen in the rings that surround it. The older they are the more rings you will see.
Great info Tom!!!! Thanks so much!!!!!!!! my baby must not have any new growth yet... Still so much to learn but I'm loving it all! I find myself on this forum many times a day! I'm obsessed haha! I can't wait to watch her grow. I may even get a friend for her one day thanks so much!!!
Many digital cameras have a "macro" function. Can you get a close up of the carapace? You can see a lot of neat stuff if you take a macro shot and then zoom in on the pic.