As suggested with Tito, it is safe to give Arnica 30C or 200C taken every four to twelve hours for a few days. I advise clients to stop the Arnica when they see improvement. This can occur after one dose or after several. But usually after a few days the animal should be better if the remedy is going to work at all. This is so the remedy can continue working and not be interrupted with another dose of remedy.
Some people think more is better.
In the case of homeopathy, this is not so. I think of the remedy action like a pebble being thrown into a pond. The pebble hitting the water is akin to the initial dose hitting your mucous membranes. Then the ripples of the water until they hit the bank of the shore are how the remedy action actually works in your body. This concept becomes more important when treating a condition with a deeper acting remedy.