Complete dilation is 10 centimeters, this is when the cervix is fully open and the baby can further descend into the birth canal (vagina) during the pushing (second stage) of labor.
In addition to how far your cervix is open will be the effacement or thinning of your cervix (measured in percentages) and the ripeness of your cervix. Effacement and dilation happen together rather than one before the other for the majority of women.
If you feel the tip of your nose (no one can see you), this would be how an unripe cervix will feel, firm but fleshy. As it begins to soften or ripen, it will become more like the feeling of your ear lobe (touch it!). When it is really soft and buttery it will be more like the inside of your cheek, very flexible and soft.
For first time moms effacement is actually more important than dilation at first, because the cervix will need to be soft to dilate. After you've had a baby previously the cervix seems to open more easily. It is said that the average first time mom will dilate just under a centimeter per hour once active labor has started and the mom having baby number two or more will dilate just over a centimeter per hour once active labor has begun.
However, there are no hard and fast rules about who will dilate how quickly that actually work. As you know babies can't read calendars and your cervix can't tell time. Some will dilate much more quickly and others more slowly.