An answer to the question Why does it take months, sometimes years, for the BEA and NTSB to complete aviation accident investigations? said,
Visiting the Site - First there will be a team that heads to the incident site to collect information. If the site is easily accessible, this may be a fairly quick process. If the site is tough to access, or takes a while to find, this will of course take much longer. This is generally the part most visible to the public, but is only the tip of the iceberg in generating a useful report.
When the site is easily accessible, what is the typical (or average) amount of time spent on site collecting data of a major accident?