I have completed several of these take-home style assignments. Personally, as a front-end developer, I prefer these to coding tests on Codality or any other tests on algorithms and data structures.
All of these have been given with specific instructions as to what is needed to be completed and how to turn it in. Most of these mentioned that they understood life gets in the way so they were really lenient with time (about a week). One or two assignments specified that if it took longer than 6 hours, don't even bother turning it in. I usually end up putting in much more than the "suggested" time, up to 10 hours, spread over a week or so. It just depends on the assignment and, ultimately, the company.