Your doctor or midwife will probably give you some idea, something along the lines of 4-1-1 or 5-1-1 (where your contractions are five minutes apart, one minute long, and have been that way for an hour)...but that can have a lot of variance in a normal labor. One mom I worked with went in at that time, only to be told in triage that she was 1.5 cm and should go home. Others have gone in at that time, found they were at 5cm, and were admitted.
A lot of childbirth educators feel that the numbers are only part of the story, and that the mother's emotional state is as important. If she is still feeling cheerful and energetic, can still hold a conversation through contractions, and is comfortable staying at home, then wait. Most women arrive at the hospital earlier than necessary, and are subjected to a lot of hospital routines that can disrupt and slow labor.
As labor progresses, the mother gets more serious. During contractions, she is completely focused on them, and between contractions she is preparing for the next one. She probably won't be cracking jokes or laughing at yours. Susan McCutcheon tells the story of one couple in Natural Childbirth the Bradley Way:
"Fortunately, they decided to take a picture of her all ready to walk out the door. Looking through the camera lens, the husband saw a confident smile and suddenly realized they were not as far along as they had thought.
This excellent coach very gently pointed this out. His wife took her coat off, and they worked at home for four more hours. Then they went to the hospital."
Of course, if you are far from your birthing location, this may be a luxury you don't have, and riding in a car during transition is no fun! However, for most people, staying at home, eating real food if they feel like it, and enjoying the last time together before baby comes is a good option.