How many years the law permits to renew your resident permit?

I 've been working and staying in China since 2012 with Z-visa which had been converted to resident permit. I don't know the exact duration (years) I'd be allowed to renew the permit too. My school wants me to work with them for about 10 years but I'm a bit confused over the maximum years you could be allowed to renew your resident permit . I have been renewing it every year. I wanna know if I can renew it every year as long as i have a job in PRC or they have specific years afterwards, you can't renew again?
Thanks