A good return is something that can't be put into a one size fits all figure. As Joe mentioned, it depends on many different things. Sure, there are returns that aren't worthwhile but that's a technical aspect and I think the 30% you mentioned is a bit out of context. What's the interest rate you get when you invest into a fund or when you put your cash into a savings account?
You may want to look at it from this angle:
If you trade to make money, why are you doing it yourself?
Maybe because it's a great way to spend many hours behind your PC while offering you the prospect of making some money with it. Or maybe because you want to make more money than what your bank is willing to pay you? If so, then you'd have to look at what a fund is returning on average. Last year they probably made anything between 13% and 25% (don't remember the highest return, it was published in the fin review a few weeks ago).
So if you want to make it worthwhile, you'll have to do better than that, otherwise you'd be better off spending your time fishing while investing your money in one of those funds.
Just one way of looking at it, really.
Happy trading
Stefan