only so long is an idiom meaning "not forever", "for only a limited amount of time". That is, there is a limit to the amount of time that the situation mentioned in the main clause can continue. In this case, "they" cannot keep their audience entertained and interested forever. Sooner or later, they will fail. The audience will get restless and stop paying attention, perhaps.
Compare:
The patch you put on that pipe will last only so long, and then it will start to leak, and you'll have to patch it again.
My father could sit still only so long, and then he had to get up and move, or go for a walk. Maybe he had "restless leg syndrome".
You can separate only from so long:
You only have your children so long, and then they grow up and move away from home.
CJ
Whoops! You weren't there, GG, when I started this post. Oh, well, basically the same thing in slightly different words. That will give emphasis to the correctness of the answers. 