I know that there won't be any definitive answer to this question, but for any particular photo project how do you know when to say enough? I've gone back to study photography as a mature age student and we are required to produce one thematically linked folio per semester which should have 10 - 15 photos. My folio last semester had the minimum 10 shots of which three or four really resonated with me and make me want to try and do more on the same theme. I have found that I work best when there is some sort of target or time constraint to tell me that I am finished, which leads me to the original question.
Say you were working on historic covered bridges, how many photos would you use if you were creating a folio (or folios) for sale? And how many more would you think that you would require for a solo exhibition? At what point do you think you would have enough for a book?
I realise the some people just keep shooting the same thing for years and fair enough. Maybe that will happen to me, but at the moment I'd like to be able to say that I gave myself (and the subject) a decent chance rather than just what i could do in six months.