Facts. It doesn't take 6 months to investigate anything, what are they waiting for? Sometimes they might need one month to check logs and third party informations since there are different timezones, times to answer, the need of personal to go and check everything. But in 6 months it's just delaying until the user gives up or they are using the money as a free loan. If you give them that much time without complaining they will take it.
Standard duration of investigation that involves 3rd party providers is up to 6 months according to reputable user here that work on casino operations.
The duration is always a case to case basis depending on the nature of concern.
However, a responsible casino will not maximize this time frame just to give a result to waiting player.
I think they say it can take this much time because some specific cases can be hard to solve, and it's better for them than giving 3 months and having to use 5 months with the user complaining that the deadline wasn't followed. But in 90% of the cases I don't understand why it would take this long.
For something simple it shouldn't take more than 3 months. I had issues where it took just 1 week for example, and more than take the casino is stalling to keep the money.