From about 4cm onwards is considered active labour. Often when women have weeks/days of pre-labour or early labour, they can have a reasonably short labour if all the hard work (where the cervix thins and effaces and dilates to 4cm) is already done. But again, there are no rules in birth, it happens as it happens. Baby's position, somestimes also their size and how active the labour is can all affect the overall length of it.