Varies from person to person, if this is baby 1 or more, position of the baby etc. Spend a lot of time on your hands and knees to ensure baby is not occiput posterior as this significantly lengthens the labor time and intensity. Avoid leaning back keep knees lower than your pelvis. Do not clench your jaw it is directly linked to the vaginal opening and the most helpful thing I can tell you is that when you breathe, do not jist fill your lungs, fill your abdomen as deeply asbpossible and breathe slowly out on exhale. Expanding the space in the uterus allows the mustles to most effectoly and efficiently do their work with less discomfort. Best to you, I did an unmedicated home birth with my second and third and used these techniques. Tremendously useful tips.
Early contractions are about 20-40 seconds, later ones might be about 40-90 seconds or longer.