The short answer is: As soon as she's ready!
To expand on that, in the absence of any complications (an infection, a brain bleed, a failure of one of her systems to develop properly) she can come home when she can do three things.
1. Maintain her body temperature without the help of her warm and cozy isolette. We're not quite there, but getting close...maybe in the next week or two.
2. Eat by mouth, what she needs to grow, and gain an average of 30 grams day. In reverse order, check, check, not yet. She's growing fabulously...she now weighs 4lbs 3 oz! She's taking her goal volume and not having any trouble with throwing up (what?? A Harms baby that doesn't throw up everything she eats?? Incredible!) But she isn't eating anything by mouth...yet...just NG tube. But she's showing signs of readiness for breast feeding and has even managed (five days in a row) to latch and coordinate a suck for 30 seconds or so, all while keeping her heart rate from dropping. What a rock star! We'll follow her cues and hopefully move into oral feeding in the coming weeks.
3. Keep her sats (heart rate, oxygenation and respiratory rate) stable without help. Check. She's been doing that for weeks now. The next step is to keep her sats up while she's eating (that whole "suck-swallow-breathe" thing is mighty tricky!) and the final test will be in her car seat or 30 minutes.
The bottom line is, Ava is doing brilliantly. She's absolutely perfect according to her doctors (and her adoring parents!) and in utero, a baby of her gestational age doesn't need to have these skills, so it's truly just a matter of waiting until her development kicks in and she's ready to do each of these things on her own. Usually that happens on or around a baby's due date (August 15) and can sometimes happen a bit earlier. So it's just a matter of time, and if she keeps up this pace, she'll be leaving the NICU and joining her sisters at home before we know it.