It’s just difficult to find succinct sources that provide that data. Justice Department investigations tend to be years in length even when they don’t involve a former President of the United States.

I think the layperson might confuse all criminal investigations with ones prosecuted by the DOJ when most crimes are actually prosecuted by state officials, not the DOJ. DOJ always moves way slower, always has.

Links below are not hard data, but it is statements from people with experience in the process:

“A Federal investigation can last upwards of 5 years due to most Federal Statute of Limitations prohibiting the Government from charging or indicting someone after that time period. It is not unusual to see an indictment that lists dates of offenses 3-5 years prior to an arrest.”

https://thetampacriminallawyer.com/how-long-can-a-federal-investigation-last/

https://www.la-criminaldefense.com/why-do-federal-criminal-investigations-take-longer-than-state-investigations