Well depth doesn't make a huge difference on its own but there are other factors at play here that influence the result. Perhaps a more influential component is the focal ratio of the system. Faster systems, say F/5 and lower will have a higher sky background flux when imaging from the burbs than a slower system. There is considerable information online related to optimum exposure times and sky conditions.
http://starizona.com/acb/ccd/advtheoryexp.aspx
http://www.hiddenloft.com/notes/SubExposures.pdf
A general principle still applies, go as long as you can before you hit the sky flux limit.