How long in your experience do developer assignments take, and how do long do the people handing out the assignments say they take?
The expected lengths for assignments always seem to assume that you won't run into any setup or configuration issues and that you've memorized the documentation for whatever programming language and libraries they're having you use. That might be a fair assumption for the developer drafting the assignment. After all, they already have their workstation set up just so, and the libraries they're asking about are the ones they're using in their day-to-day. But that's not the case with your machine and your brain, so it's going to take you longer.
It is like IQ tests. The more you take the smarter you get. But then again not really...