In practice, I think the amount of work will probably vary depending on the subject, institution and the effort wanted/needed.
In theory, under the CATS Scheme ( which is what uni's use to work out how to transfer credits between institutions) 1 credit= 10 learning hours and Masters usually require 180 credits in total. So a Masters should take 1800 hours to complete.
But even with on campus Masters there is a large proportion of independent study so some students will do much less than this and some more depending on how easy or difficult it is for them and how much effort they make.
I am interested in doing an MSc by correspondence. It runs over 2 years and it seems to accrue "points" as you go along, 45 points in the first year, 60 in the second. But I work FT and have four children at home. I also run a small clinic from home and often have to write up reports on the children I see. How much study is involved?