This is how I do it, as a Director dn hiring manager. 4 hours max, and the exercise is entirely freeform against a set of 10 typical requirements; as a Front End Engineer candidate for my team, I want to see your opinion and execution matched to the problem at hand.

It can be done in 2 hours in the most simplistic, undeliberated way using legacy practices, but 8 if you're a perfectionist. Come in somewhere in the middle, but stop at 4 hours no matter where you're at.

I don't even require that the exercise is completed, but I want you to speak to your thought process and where you went/were going when you hit the 4 hour mark. The ability to understand our space, make good decisions, and show progress toward executing on them is ultimately what matters in the end.