How did you know you were "done"?

I'm 36, have been happily married for 8 years, had 4 kids within 6 years, work full time, and yet I still want more. Am I crazy?

Ever since youngest ds was born I have felt that perhaps eventually we would expand our family.  I always imagined myself with 5 kids. My husband wanted 3. Our fourth was a surprise, I should probably count my blessings. 

We have the room in our hearts and the finances to support another mouth to feed. Why shouldn't we go for it? I have talked to hubby and he is not exactly on board, but he said if it happens, it happens.

My question to  you ladies is how you knew that you were done. Do you ever get past the desire to have more? Or is that a sign that you should?