In reality, any kind of serious investigation will likely take anywhere from four to six months.

First, they need to contact external companies, solicit bids, and decide which company to go with. Depending on response times, the scope of the investigation, and how many companies the corporate bean-counters are requiring, this by itself could easily take a week.
Next, they need to actually schedule time for the company to perform the investigation. This could easily be two months away, simply so the company has time to wrap up any of their current work and devote employees/resources to the investigation. The company can’t just drop all of its existing contracts to take on a new one, and I can guarantee they’re booked out at least a few months in advance.
Third, they need to actually perform the investigation. Depending on the size and scope of the investigation, this could take anywhere from three weeks to three months. Even just performing a one hour interview with every employee could easily take six to eight weeks if it happens during regular business hours. And this is likely going to be a fairly large investigation with dozens of employees to interview, paper trails to follow, social media to dig into, etc…
Lastly, the company needs to compile all of their info into a report. Again, depending on how much data there is to sift through and how broad the investigation is, this could be anywhere from a week to two months. If the allegations are anything to go off of, the report is likely going to be a full spiral notebook worth of info once it’s condensed, so it’s not something that can just happen overnight.