A university would state that for a 10 credit module you are expected to put in 100 hours - split between lectures, labs and private study. Thus, in a semester you will cover 60 credits, thats 600 hours in a semester. Over a 12 week semester thats 50 hours a week. I think 60 hours a week is too much.
Treat your studies like it was a job - if you were working full time, you would be required to work 37 to 40 hours a week. That includes 90 minutes for lunch and breaks. Thus 6.5 hours over a 5-day week or a little over 4.5 hours over 7 days.
20 hours a week seems light but hey, everyone is different. if you are getting marks in the 60s and 70s then no need to change.