When I arrived at hospital they recorded the start of my labour as being when I had regular painful contractions. For me this was 6 hours before I got to hospital and I was 6cms dilated at that time. I gave birth 11 hours later so they called that 17 hours of labour. Before the regular contractions I'd had about 18 hours of pre labour as well.

Have a look at your discharge papers (or your baby's) and you will see how long they classed your labour.