Employment Law Questions? Ask an Employment Lawyer.
The federal laws put no time limit on how far back a criminal offense is reported, so if it exists it can be reported under Federal law unless and until it is expunged (some offenses can be expunged from your record after a certain period of time....this is a state by state, offense by offenses determination that you should seek assistance from a local trial defense counsel to process).
Now, that being said, many states have passed their own laws limiting the reporting of some or all criminal convictions that are 7 years old or older. You didn't list your state, but if you want to ask a follow up using REPLY, I can check your state specifically. Of course, the limitation is on the reporting agency, so if your employer uses a background check company in a state without restrictions (and many employers do use outside agencies), then the limitations imposed in the state where you live won't really apply.
Ok. Where you work and the location of the offense are not relevant. What is relevant is the location of the company that conducts the review.
As I suspected, it appears that Illinois was chosen for a reason (and why that background check company operates there). They have no limitation on background checks for criminal convictions. They can look back for as far as there are records.
If you're talking about the search company, they look at everything. It would be more work to limit their search. They just pull up the information on a computer database and everything comes up for them.
As for the employer, unless they have some sort of policy where they personally impose on themselves a requirement to not consider anything older than 7 or 10 years (whatever they may choose), they also look at everything.
I couldn't tell you what an individual employer chooses to do though. I would operate from the assumption that both the reporting company and employer will look at everything there is to see. However, if you've been a valued employee for this entire time, the employer can (and usually does) take that into consideration also.