If I had to put a tangible figure on it, I guess I would say anyone who has passed JLPT N4 would be able to use the combination of their knowledge and the furigana to make it through this kind of manga without getting completely stuck. They might still need to ask questions, but they would probably be infrequent enough that it wouldn’t feel like a complete drag. It would be slow going due to having to go to the dictionary a lot though.
Any higher than that and obviously speed and comprehension would improve.