The answer is, in my opinion, "anything you can cast with it". There's no rules to this. That being said, at some point the flies will be too wind resistant or heavy to cast WELL with the setup, and using it to throw #18 blue winged olives is a bit overkill (careful when setting the hook with 6x), but there is nothing to say you cannot do it. An 8wt is capable of throwing some serious flies in the right hands.

Take it out and try it and see what works for you. After all, you will be the one actually doing the casting, and thus the only opinion that really counts.