RescoUSA-Made Nail Grooming Kit, Large, Yellow (PF0531)

Q: Will these work on 200 lb English Mastiff?

I don't see why not. I have two dogs... one has thick black nails (wherein you can't see the nerve endings) and one of my dogs has clear white nails. I never had a problem with this nail clipper but have never clipped the nails of a 200 lb dog before. Wish I could be more help.

Q: How often/after how many trims does the blade need to be replaced?

Hello, About every 4 to 6 months. It all depends how often you use it and the breed of dog. I have a lab and his nails are thick and long. I cut his nails every 3 weeks. Hope this helps.