How long did you wait before sleeping over at your boyfriends place?

I met this wonderful girl online a couple of months ago and we have been on six dates since then and recently become boyfriend and girlfriend. We live about 1.5 hours away from each other and each time we met at the weekend would spend several hours together. I told her she was more than welcome to spend the night at my place on the third date, but she declined as she said it was too soon. Should I ask her again now we're officially a couple? I'm not looking for sex, would just like to spend more time with her. What would you say to her and how long was it before you stayed over at a boyfriends place?