What Should Be The Distance Between Placenta And Cervix During Pregnancy?

Hello Dr, I am 29 weeks pregnant and was diagnosed with low lying placenta during my 24th week. The placenta was 17mm from the cervix. Later it had ascended to 24mm in the 27th week. I was advised bed rest since my 24th week. I would like to know what is the safe distance that the placenta should be from the cervix, so that I can resume my work? Regards, Varsha