Or do you think 3 days isn't enough to help?
I really don't know, Dee. I'm not sure how long those treatments stay in the body, or how long it takes the liver to get rid of it. Initially, I was taking into account that those products work up to 3 months (I think?), so maybe give the milk thistle for a month, but since you don't give it to the boys very often, perhaps just give the milk thistle for 1-2 weeks. Unfortunately, I haven't found anything that specifies how long you should give it. This is where a holistic vet like Dr. Hofve would be able to advise. I wish her article specified what she means by "long-term."
I do have a book (from 1999, so not recent) about natural remedies for cats that mentions milk thistle in several places. It doesn't mention anything about not using it long term. The longest period I did find in it, mentions 3-4 weeks, or for a cat on seizure medications, for the entire time that the cat is on the meds. However, this book is 15 years old, so I take what I read in it with a grain of salt.
Sorry, that probably doesn't help you much . . .