Pingueculitis from time to time, even with artificial tears. How much and how often should I take dexamethasone? Is it bad in the long term? Why? Thx!

Some risk. Dexamethasome eyedrops have a small risk of glaucoma and cataract formation. It might also promote the growth of some microorganisms causing infection. But you can use it for 1-2 week bursts with little likelihood of harm. You might ask your ophthalmologist for a steroid with more limited side effects such as Lotemax (loteprednol) or fluorometholone which can be effective against your inflammations.
Steroid side effects. Dexamethasone is a steroid and there are side effects of long term use of steroid eye drops. Some of the more common side effects include high pressure in the eye which can lead to optic nerve damage (glaucoma), and clouding of the natural lens inside your eye (cataracts). Long-term can mean as short as weeks of use, to as long as many years. It varies per individual.