hi.. how old is she now?
if you're already leaving her, she must not be too young..on the other hand, if she's staying by the door or in the crate, maybe she is pretty young.
When i did it, i didn't really have any particular age guidelines. i read in a puppy training book that you gradually increase it, as you are already doing, to see how long they can be trusted. Of course, you puppy proof the house so that there's nothing they can get into or destroy. You wait, of course, until their housetraining is pretty reliable with only the rarest misstep.
I think Zack was around 8 months old when i started leaving him free in the downstairs part of the apartment while i was at work, or sleeping upstairs during the night. It could've been older, but that seems about right. I kept the upstairs gated so that he didn't have access to the cat box or the cat's food. I started just leaving him out when i'd do errands. Then i started leaving him for about 3 or 4 hours while i was at work and would come home for lunch. There was never any problem with it. Once in a while, he'd chew a ballpoint pen or some little thing, my fault for leaving things around, but generally he would leave them alone so i'd not think about it, he never peed or pooed in the house, he would just lay in once place all night when i was sleeping, but he would do some playing while i was gone during the day, he has lots of toys.
At some point, i started leaving the gate to the upstairs open so the cat could come up and down while i was at home, and that was OK for many months. Then one day, he had catbox breath, it was the worst and it lasted for a couple of days. :yikes So now i keep that gate closed and just bring the cat out when i'm here.
Before i started leaving him out, i transitioned him from just the crate, to the crate surrounded by an xpen in the kitchen, to just being in the kitchen with baby gates, and then finally, free in the downstairs. He hated being put in the kitchen so much, he acted depressed. I was happy when i was able to leave him free in the house.