You have gotten a ton of good responses, and it seems like people have addressed the water breaking issue, but I just wanted to throw my two cents in about how long to wait. I just want to point out that every labor is different, and I think it's important to have a plan about what will happen if you wait TOO LONG. If you plan to wait until you're in transition or pushing, the possibility is there for things to go too fast. Will you jump in the car even if you're already pushing, or completely out of control from the pain of transition contractions? Will you call 911? Will you prepare (really prepare, not just briefly entertain thoughts) for an unassisted birth just in case? Have a plan.
My experience was that my contractions started after sex. I always got crampy after sex, and when I realized the crampy was rhythmic this time (every 3-4 minutes from the very start) I called the nurse at my hosp. MW practice. She told me to get some sleep and come in in the morning. I did, I slept through almost my whole labor. My contractions never got stronger, longer, or closer together until transition, which was about 5 minutes worth of totally crazy strong contractions before I started pushing in my bathroom at home early the next morning. I didn't have a plan for this, and I was terrified and not able to even think, so my husband drove me to the hospital and I pushed out my son within 15 minutes or so. It turned out to be a very successful strategy for avoiding pain meds in labor, if that's your goal!
However, it sucked as a strategy for a peaceful birth, for feeling in control and not scared, and if you don't want complete chaos surrounding the birth and immediately afterwards. This time I'm planning a home birth, but if I were going the hospital route I would either get a doula and leave as soon as I was sure I was in labor, or I would prepare for an unassisted birth, try to time things, but plan to stay home if I waited too long. Or both
it pays to be prepared. There's no way I'd want to go to the hospital in transition or pushing again (barring an emergency of course).
Just my two cents.