Hi Pipermaru, Welcome here, really nice 1st study. Your shapes look good. Only thing I have noticed is, his sleeves near the hands are very bright in the original, especially on his right . Yours might need a little bit of adjustment. You are missing some texture in your copy too, but this is your first one so its best just to get familiar with photoshop workflow first and later get into textures. Otherwise really nice work !
I too spend a lot of time on these, on average 7 hours a study. I believe that your speed will improve but the question is when to stop. I think you are always learning regardless, sometimes -I believe- its good to do a longer one to say invest time into eg learning how to paint the hair, or certain texture, or se how far can you go with precision... I learnt from these longer ones a lot. But at the same time, I don't want to spend half a year on these so i'm choosing balance of longer studies 10+ hours and shorter ones 4 hours, one hour is never enough for me personally, I found it takes me at least 4 hours to be sort of happy with it. But this is just me.
Zbrush anatomy sounds great ! Is ZBrush difficult to learn? I actually don't even need to learn it I would like to get model of skull in ZBrush and use it as reference. Being able to rotate it would be a great tool for studies.
Anyway I'll be looking forward to see more from you man ;-)