Take home interviews are common in data science, and they are deeply exploitative. I have been asked to spend anywhere between an hour to a week on a project, and in most cases they simply decline to hire you without giving you any feedback. In the worst cases, companies like Knewton and Mattermark have given take home interviews that not only took a lot of time, but were related to the companies core business model. In other words, it's free consulting.
And finally, you aren't fooling anyone when you say that it should take 3 hours, or as long as they want to spend on it, while giving them 3 days. You are pressuring people to spend as much time on the problem as possible to "prove they are a good programmer." It's exploitative - you aren't paying us and the probability that we will get hired is extremely small. Think about how many companies we are interviewing with.
And finally, you aren't fooling anyone when you say that it should take 3 hours, or as long as they want to spend on it, while giving them 3 days. You are pressuring people to spend as much time on the problem as possible to "prove they are a good programmer." It's exploitative - you aren't paying us and the probability that we will get hired is extremely small. Think about how many companies we are interviewing with.