Hi Otto!
In the end, i did 17 interviews, ranging from 20 minutes to 3 and a half hours (to be fair, that one wasn't really an 'interview' at all - the interviewee hijacked it, never really answered the questions and i don't think i dealt with it well). The average length was about 40 minutes - with experience, i did find ways of getting the interviewees to elaborate. Nevertheless, none of my supervisors thought that short interviews would be problematic, given that the quality of the content was relevant & good. Also, i still decided to stop at 17 interviews as i realised that i had plenty of data for analysis (although i haven't started this yet!) and feel that there are ethical concerns regarding 'over-collecting' data, and then not using it.
Hope that helps - give me a shout if you need to know anything else!
Best Wishes, Z