My water broke when I was at 2cm. I was 38wks & 1 day. I wouldn't dilate anymore though so I had to be put on pitocen. But my cousin was 2cm for like a month, so don't get your hopes up.
I was at 4cm for the last 6 weeks of my last pregnancy. I went over.
I say you have anywhere between 4 hours and 4 weeks. There is no way for us to know.