Of course the answer is "it depends!"
What sort of fish are you going to have in there, and do you intend to have live plants?
The old adage "you can't have too much filtration" is true, you can't! However your fish won't appreciate a 100mph current in the tank either
As a general rule you want to have 5-10x turnover per hour. So for 30 gallon you're looking at a max 300 gallon/hour turnover. However manufacturers usually lie about their filter rates so assume 50% of the value.
So you're looking for a filter which claims 300-600 gallons per hour of output (1000-2000 litres in understandable language
I've learned this the hard way in getting an external which claimed 700 litres/hour but only does 360!