The IRS and other agencies start to consider a temp employee permanent, and eligible for all benefits, after about 12 months.

There is no legal time limit. As long as you are w-2 you are technically an employee of the temp agency. I worked for 3 years for Pepsi before getting fed up and finding a real job and I've heard of others who were temps for over 10 years. It is becoming a very pervassive for of labor abuse especially in STEM professions and there is nothing that can be done except keep searching for a perm job and quit when you are offered one.