Is there a typical time on site during an investigation of a major accident?
There are simply too many variables to answer this question, even in general terms.  And wreckage accessibility is just a small part of it.  

If an aircraft runs off the end of the runway and the pilot survives to make a statement that can be corroborated by another crewmember, tower and other eyewitnesses, as well as weather reports, it doesn't take much time to wrap things up.  Probably a few days is all that is actually needed on-site.  Most of the work at that point is administrative in writing the report.  The aircraft may even be mostly intact and towable.  

However, when all you have is a smoking hole in the middle of a pasture, as accessible as the wreckage may, be the investigator's workload is massive.  Sifting through all the broken pieces, numbering and cataloging them, plotting locations in the debris field, etc. can take months in itself, and then the real investigation begins.  Not to mention the cleanup.