At the moment I do it full time and so of course I have the time to dedicate that much time each day on writing. If you are already studying or working, then of course you should rightfully treat it as a hobby, side job, extra activity or whatever put firm limits on how much time you spend on writing. Spending six hours a day on writing when you already have an existing full-time job or study is insane.
The only other thing that I can say is that readers aren't interested in your own circumstances. Like audiences of a theater, they only see a carefully crafted show. They don't see what happens outside the performance date and off the stage and how much effort the actors put into practicing and mastering their performances.
This is the nature of art and the entertainment business. Readers are only presented with a finished product that is the culmination of our hard work. It is unfair to make a comparison between a novel written by someone who writes fulltime to a novel written by someone who is writing part-time. Yet readers don't know any better or don't care and make those comparisons anyway. This is the reality all authors have to live with and it is best not to listen to these kinds of voices at all.
All I can say is that passion and enjoyment are vital to sustain writing in the long term. Write as much as you enjoy and your free time allows and no more if you want to keep it up for years. Writing is not a race nor a competition to be the fastest writer or the best earner, though all the comparisons and jostling for power stones something make it seem like it sometimes.