Maybe 5 cm? Every Dr is different. with my first born I was 4 cm at my ob/gyn appt & she had me to go straight to the hospital.... I now see a different ob/gyn & she won't admit you if you're 4 cm.
you have to be in active labor usually. either with contractions and progressing in dilation or your water has broken. also depends on the place you're at as well though.