How far back can an employer go back on a criminal background check?

Unfortunately there is no time limit for how far back an employer can check into a criminal record.  Your record is your record and you have one.  Since you can neither seal or expunge it (I assume you have checked on this) perhaps you can get a "Relief From Civil Disabilities" if Colorado allows for something like this (NY does).  This will not clear your record but makes it so that your it cannot be held against you for purposes of employment and such.  In other words, if you were not hired it would have to be for another reason other than your criminal record.  Also, some states allow for a felony to be reduced to a misdemeanor even after conviction.  You would still have a criminal recored but if only asked about felony convictions, and your charges were reduced, in that event you could answer "no".  Also, some companies will still hire you with a misdemeanor record.  You need to consult with an attorney in your area as to all of this.