How long can one leave sample in lysis buffer for DNA extraction?
It should be fine. Unless they have been contaminated with DNase, none of the components will damage or denature DNA. I've left purified plasmid DNA on the bench for weeks with no apparent degradation, so you should be okay (not that I recommend doing that on a regular basis). I would continue with the purification, according to your professor's recommendation, because you've already come this far. If worst comes to worst, you won't get anything, and you'll have to re-purify. However, the chances of your getting something are pretty high, so you may as well go with it.