I assume you mean hetero boy on girl straight sex. Top or bottom, front or back, it's all good. It's just different is all. Whatever feels good; let go and go wild and men don't seem to mind, I promise you. It's almost as though their penises were just made to fit snugly inside our vaginae, and that's a wonderful thing.

I like being on top for better control*, but I love it better when he's on top and takes me. I love to move (music is excellent for this), I love to moan and whimper and bite and scratch and writhe and squirm around and gyrate and wrap my legs around him and feel him inside me, stretching me to the limit. Depth hasn't been a issue. Tightness was an issue at first, but what man doesn't love a tight ...?

Besides, a hard penis stretches a vagina way better than those old hard plastic dilators. Let's just say in regards to penetrative sex, as with all things practice makes perfect. I personally try to get as much practice as possible.
As for how many weeks, I reckon just as soon as you can.
*by on top for a girl, that's straddling his penis with it embedded inside you. You can then lean forward and kiss, he can easily access your breasts, or you can lean backward, squirm around, up and down, back and forth, or just let him impale you to the hilt.