Hi v bodani
With regards to my experience level, I have 8 years experience in IT, predominantly as a senior developer. I have about 3-4 years experience as a java developer and have been involved in architecting a few applications with a team of architects. My experience also spans the Microsoft realm with ASP, VB and COM experience.
Regarding how long one should spend on the assignment, I don't think the quality of the assignment is necessarily related to how long you spent on it. I know that I sometimes had to wait for 20 minutes for some of my diagrams to save!! I probably spent about 40-50 hours on my assignment but I was only spending 1-2 hours a week on it for some weeks. I think I could have finished it faster if I had been able to spend a day or two solidly working on it.
I am thinking that perhaps I lost marks on some of the assumptions I made rather than the actual technicalities of the diagrams.