Yes. If you did nothing but study from sun-up to sun-down for 15 days, I think you could get through all that is required. For most of us though, that is just not realistic. The interview should be your sole focus, but the reality is we still have family/work commitments. It took me about 21 days to get through everything, and that was meeting or exceeding the recommended time spent on each RST “day”.
It’s a great exercise in time management. I’m stating the obvious, but RST only prepares you for a portion of the interview. If you are doing other prep services, that can account easily for another several hours a day working on that stuff.