I declined to participate in an exercise similar to this while looking for a job this summer. The task was pretty complex, complex enough that I knew it was going to take a significant fraction of my weekend, but doable.
I live in Colorado and if you think I'm going to spend the weekend working on a job offer instead of heading out for the activities I want to do, you may be surprised to get my email saying no thank you.
tl;dr some people don't wanna do your interview on their weekend so be flexible.