Earlier in the year I took advantage of a deal that Microsoft was promoting regarding a voucher for a free examination. Even though I initially planned to study over the course of several weeks, I ended up only being able to truly study for about 4 days or so. I would say that I averaged about 4 hours or so per day which comes out to 16 hours.
If you’re looking for a great guide on the topics covered, use http://www.bloggedbychris.com/2012/09/19/microsoft-exam-70-480-study-guide/
The videos that Microsoft has up at http://www.microsoftvirtualacademy.com/tracks/developing-html5-apps-jump-start are also useful.