There is no need to give additional supplements. If piggies have a good balanced diet then they get enough vit c from their hay, veggies and also pellets are fortified with vitamin c. They do not need routine supplementation of any kind. A short course (a couple of weeks) of additional vit c can be given during a period of illness as a boost to the immune system.
Excess vit c is simply excreted out in the urine, so if your piggies have a good diet, then you are simply wasting your time and money by giving additional vit c as it’s just coming straight out the other end.
At worst, it can cause problems. We do not see many cases of scurvy in piggies on here and those very few cases we do see often come due to the use of supplementation. Long term supplementation can mean their bodies get used to abnormally high levels of vit c. If those levels then drop from what their bodies have become used to, even if the level which they drop to would otherwise be considered within a normal level for a piggy, then you can see scurvy symptoms occurring.