Beyond that, I think "a few hours" is a bit too much to ask for, especially since you are presumably following up with an in person interview centered around the assignment. That's a big time commitment, and the commitment on the assignment especially is very asymmetric.
I'm not a big fan of assignments for this reason. They are just so asymmetric. I've been given interview homework that was supposed to take "about an hour" and it was more like 4 to 5 in reality. It felt like an unreasonable time request. I did the assignment, and did in fact tell them it took considerably longer than their resonates. I was invited for an on-site after all that, but declined for other reasons.