The work put in by students in England varies greatly between both subjects and institutions - bringing the value of a degree into doubt, a report says.
Some universities award more higher class degrees than others
For example, those studying medicine at the University of East Anglia worked for 45 hours a week - compared with 29 hours at Queen Mary's in London.
The survey of more than 15,000 undergraduates was commissioned by the Higher Education Policy Institute.
The government said rigorous testing meant hard workers got good degrees.
The report also says some institutions award many more first class and upper second class degrees than others.
"Explanations for this might be that the students concerned are more able, or else that they work harder," it says.
"On the basis of these data, neither of these explanations appears to provide a complete answer.
"The report does not prove that the degree classification system is flawed, but it certainly raises questions that need to be addressed."
The study follows the Burgess report which called for an overhaul of the degree classification process.
The vice-chancellor of the University of Bedfordshire, Les Ebdon, said the survey showed that different subjects required more teaching time than others.
But he argued the sample size used for each individual subject at different universities was often too small - down to five students in some cases - to make direct comparisons between institutions.
He told the Today programme on BBC Radio 4: "We actually go, in this country, to extraordinary lengths to make sure there is comparability between degrees. The external examiners' system is famous throughout the world - we have a Quality Assurance Agency that does that."
Students reported an average of nearly 14 hours of scheduled teaching a week - and claimed to attend 92% of these.
This varied from around 22 hours for those studying medicine to around eight for students of history or philosophy.
Private study was also taken into account. Architecture and law required the most, at around 17 hours a week.
Scoring least well - and also low down on the hours of scheduled teaching - were subjects such as media studies.
The total number of hours put in, for both scheduled and private study was 20 per week.
Students were also asked how satisfied they were with the amount of teaching they received. Those with low levels of teaching were most likely to be dissatisfied.
Overseas students - who pay higher tuition fees than British students and those from other EU countries - were more likely to say their university experiences did not represent value for money.
Universities UK president Drummond Bone agreed that the variation in degree classes raised some "interesting questions".
But he said there was no national curriculum in higher education so there should be no surprise that there were variations between courses and the number of teaching hours was not an indicator of the quality of education.
The director-general of the Institute of Directors, Miles Templeman, said employers looked for many qualities when employing graduates.
But he said: "There should be a common sense of what a 2:1 means or a first class degree means across the country, but degrees are just one measure of what we want."
The University and College Union said the survey reflected the pressure academics were under to commit time to research and administrative duties.
Joint general-secretary Sally Hunt said: "Both students and staff want smaller class sizes, which comes as little surprise when one considers the student-staff ratio is now higher in universities than in our schools."
Higher Education Minister Bill Rammell said institutions needed to respond to the demands of both students and employers to produce graduates with the skills needed.
"Rigorous quality assurance and testing ensures that only those who put in hard work get good degrees," he said.