In general, yes you have to keep working for the same employer for 2 more years after grant, that count down starts from after the pr is grant, no matter how long you have waited for and worked there before grant.
But, as previously posts some people suggest that after you got your pr, they won't usually do anything about it even if you leave the company within less than 2 years, unless the company reports you and proves that you had no intention to work there for 2 years and gave false information when you were applying for your pr, then they might do something about it.