Our experience is that when your survey has reached 200-400 responses our clients typically can see a clear trend on which tasks are demanded and start to see Completion Rates for the top tasks.

For a single task, our clients often see trends emerge after 25 - 30 responses. This is especially true for seeing why customers are not able to complete their intended task and what they intend to do instead.

Another aspect to this question is the length on your Task List. Complex sites with long Task Lists often need more responses to provide actionable results.