The word ‘only’ here shows that the amount of time is limited. For example: “I can hold my breath for only so long before I pass out.”

Consider this sentence: "You can only hold your breath for two minutes." The meaning is clear. However, there is a problem. Some people can only hold their breath for a minute, some people can hold their breath for five minutes. So it is not true that "you can only hold your breath for two minutes." We know that there is a limit. You can't hold your breath forever. How can we express the idea that there is a limit, without naming an exact length of time? We can say "You can only hold your breath for so long." Here "so long" is a kind of placeholder, like the letter X in algebra. It means you can't hold your breath forever, there is a limit--but we can't say exactly how long it is. In your example, the writer is saying: a) There's no progress in the plastic arts. b) The lack of progress can be hidden by religious painting. c) The lack of progress can be hidden by religious painting--but only for a while. d) Religious painting can't hide the lack of progress forever. It can only hide it for so long.

Your questions was very interesting!