I've been working at a software development firm since January. It's my first job out of college. The last few months have been particularly busy for me (50+ hour weeks) and I'd like to take maybe a Monday off so I can take a three day weekend at some point soon. 

Is it inappropriate to begin thinking about vacation at this juncture? I have more than enough PTO saved up, I just don't want to cause any trouble with my employer. So far they seem to be pretty flex about working hours. Since I'm just a junior developer I don't know how it would look.