The length of labour used to be regarded as 'normal' if it lasted 18 hours. In recent years, this standard has come into about 12 hours. Generally, the prelabour and the early phase of the 1st stage do not come with preset time limits, (unless the waters have broken). This is because the labour is not 'established', and therefore not regarded as having truly 'started'. When the labour moves into the active phase of 1st stage (where the cervix has opened more than 3cm), caregivers will tend to be more focussed on how quickly the woman is progressing.
What defines 'adequate progress' will vary from caregiver to caregiver, and hospital to hospital. As a guide, the traditional medical definition of what constitutes progress in the active, and transitional, phases of the 1st stage of labour is about 1 centimetre per hour for first time mothers (this is called the 'Friedman's curve'). It is expected to be less than this if it is the woman's second or subsequent baby. The 2nd stage (or pushing phase), is aimed at being completed within an hour or two for first time mothers, and 1/2 an hour, to an hour, for mothers having their subsequent baby. In recent years, more caregivers have relaxed this definition being more flexible in accommodating individual differences (although, this is not the case for all caregivers). Adequate progress should be more in line with the woman actually making some progress every few hours, rather than a measured scale of progress that her body should follow.
While it is very normal for several women to deviate from the so called textbook 'norm', in reality, not many are given the luxury of giving birth in their own time. Women are individuals, and as with any natural process, labour will vary, and yet still be 'normal'. Different hospitals and caregivers will have their own interpretation of what constitutes a 'slow labour'.