Trends on time spent studying over the the past few decades, for students outside the US
For Germany the data is there in the form of the "Studierenden survey", which exists since 1982. However, I don't know if it is analysed. The datasets are available, so I added a list of links to the datasets for the different years in case you want to do the analysis yourself.

2012/13: http://dx.doi.org/10.4232/1.5126 

2009/10: http://dx.doi.org/10.4232/1.11059 

2006/07: http://dx.doi.org/10.4232/1.4263

2003/04: http://dx.doi.org/10.4232/1.4344

2000/01: http://dx.doi.org/10.4232/1.4208

1997/98: http://dx.doi.org/10.4232/1.3511 

1994/95: http://dx.doi.org/10.4232/1.3131 

1992/93: http://dx.doi.org/10.4232/1.3130

1989/90: http://dx.doi.org/10.4232/1.2417

1986/87: http://dx.doi.org/10.4232/1.2416

1984/85: http://dx.doi.org/10.4232/1.1885

1982/83: http://dx.doi.org/10.4232/1.1884