So, how little data are too few?

Don't use percentages when the data you have are too few. If you have 3 data points then saying "%67 of reviewers gave it 5 stars" is both accurate and misleading. It is better to say "2 out of 3 reviewers give it 5 stars." Doing this assists the reader's intuitive grasp of the usefulness of the rating. When I was following the local school district's quantitative heavy presentations it was obvious when they chose to use percentages and when they chose to use counts in order to give a positive depiction of bad news. Don't cause your reader to doubt your trustworthiness.

So, how little data are too few? I would say anything less than 100 data points.