I highly reccomend going in immediately. If it hurts, it means it is infected. If it is infected, you can get blood poisoning, due to the high number of blood vessels in the mouth. It is not rare, and it can be lethal. Dont wait.
I know from personal experience that I can wait 18 months. I already have and I will continue to wait until I have saved the money I need to pay the dentist. It hurts from time to time but I manage to keep it clean with frequent brushing and by rinsing with warm salt water.
You could wait if you absolutly had to but I wouldn't suggest it. Without the filling your tooth is more likely to continue to corrode away and get more cavities. When you have a hole in your tooth food gets stuck up in the hole and stays in there. That causes the corrosion to continue at an alarming rate.