How did you know it was time to have a baby?

Dh and I have been married for a little over 4 years, we are starting to get the "When are you going to have a baby?" question -- (A LOT!)

We have been moving around a lot for dh's job durring the summer months. And I am so close to being done with school, that I am feeling that right now isn't the best time for us.

So my questions is... How did you decide it was time to have kids? How long did you wait, or was it a suprise?

Thanks.