I'm with loveitlong... to the bottom of her shorts... those thin ends will only get worse...

a nice blunt cut with a slight U, then regular 1/2" trim every 2-3 months... and you will both be pleased.

Just remember that NOT everyone CAN grown hair to the floor...

I'd be completely happy with her length so long as the ends were thick.

Enjoy it pal.... you're a lucky guy