We're talking about temporary employees, co-employed through another agency and not contractors, right?
If we're talking about temporary employees, I don't think there's any time limit. You ask about discrimination and therein lies the concern. It's not that using temps or keeping people in temporary status is, in and of itself, illegally discriminatory, it's that you have a concern that something about your practice may be illegally discriminatory that raises an eyebrow. Does your company, for instance, tend to hire people into direct employment for the very same jobs that you have people performing on a temporary basis (for less pay, benefits and/or opportunity for advancement and/or training)? What is it about using temps over the long haul that seems discriminatory? I agree with Ruth that you are probably flushing money down the toilet by keeping people on a temp basis that long unless you have a really nice (i.e., expensive) benefits package.
If we're talking about contractors, you may well have a serious problem but it's not discrimination, it's FLSA and benefits administration.
Probably one of the biggest concerns that arise with maintaining Long-Term Temps is the co-employer issues. This occurs because not only is the temporary company an employer but your company is one as well. There have been many cases litigated based on the co-employer issues. The exposure is just as real as you actually being the sole employer of the temporary. I have attached a web address to an interesting article you might consider reading.
Another area I would suggest you looking into is your company's General Liability policy. You will want to check the section that talks about "Who is an Insured" in order to clarify that your temporary employees would fall under the definition of who the carrier is willing to cover. Often times, General Liability policies will exclude temporary employees. If you have a temporary for a short amount of time and then hire them on full time then your exposure is manageable and at least limited to that period of time. However, if you end up keeping temporaries for extended periods of time it becomes an ongoing exposure.Hope this helps.