Great question, there are so many factors involved that can speed up or likewise slow down delivery of your baby
1. If it is your first baby it is more likely to be a longer labour as your body has never laboured before, needless to say there are some women, including yourself and me that delivered your first baby within 4 hours, you are the more likely you are to deliver normally if you have looked after yourself prior to getting pregnant.
2. Position of your baby -
When you are in labour the baby can only come out vaginally in two ways with the head down, the majority of babies are positioned head down and tucked in so that when baby is delivered it is facing down and the head presses on the cervix evenly so that it dilates evenly.
There are some babies who decide to come out facing up, which is called an 'OP Position' this means the baby's back is laying on your back, or a back labour. This is very difficult in labour as it is more painful for women and lengthens the labour further. The reason for this is that the head presses on the cervix but it may well be unevenly dilated, the presenting part of the head is coming down in a slightly wrong position causing the cervix not to dilate effectively, if the baby's head is positioned left or right the diameter of the head wont come through pelvis.
Also how far engaged in the pelvis is a factor as a woman whose baby that is not engaged properly can go into labour but it may go on longer if the head is not descending onto the cervix.
3. If you have had babies before vaginally you are more likely to have a quicker labour the next time around as your body has been through it all before.
4. Position of the mother - if a woman keeps herself more upright in labour then gravity kicks in and helps with the descent of the head.
5. Strong regular contractions, lasting over a minute each and having 4-5 contractions in 10 mins - the force of the contractions should be strong enough to push baby down, if they are not regular then the baby is not going to descend into the pelvis the combination of these, the baby's position and descent of the head, the presenting part of the baby on the cervix and the mothers position are all factors in having a normal labour.
Some types of pain relief can slow down or delay a labour such as an Epidural as it takes away the pain and the feeling of wanting to push, however there are times especially if you are having a back labour that an epidural can be beneficial.