When will my boobs drop!? I am a week post op. I started at an A cup (no sagging). (Photos)

As your implants are exceptionally large, I would expect it to be 4 to 6 weeks before you begin to see some settling. It will likely be 6 months or more before you can judge the result. I hope this size works out for you. 

Thank you for your question.  Anxiety about high implants is common in many patients and this forum is filled with questions related to this every week - so you are not alone.
Some of the high appearance in the first 2-3 weeks is due to swelling of the Pec Major muscle over top of the implants.  This exaggerates the appearance of a high position for the implants.
Implants typically drop over 6 months with saline dropping more than gel and smooth more than textured in my experience.  In the end an ideal breast has an upper pole to lower pole ratio of 45%/55% and the lower pole has to stretch over time to create this shape.  As someone who started with an A cup, you might have originally had a nipple to fold distance of 4-6 cm.  This has to stretch to 13-16 cm to fit the 900 cc implant.  The stretch in the tissues takes time and there are a lot of factors that will influence where the implants ultimately settle (your tissue elasticity, activity level, sleeping posture, degree of surgical release of the supporting structures at the time of the procedure among others).  The larger the jump in volume with the initial implant, the longer it takes and the less predictable the final position is.  In my practice I often recommend two stages (smaller ~500 cc implant first and then bump up to final volume) but every surgeon has a different philosophy, skill and experience in this.
I would see your surgeon regularly so they can monitor the implant settling over time and help balance your use of supportive garments with your activity level to try to optimize things over time.  At one week    things look pretty much as expected, given the surgery that you had, and it will be better in another 2-3 weeks as swelling goes down and the tissues start to stretch.

Thank you for your question and photographs.  My strong advice is to wait a full 4-6 months before considering the result of your breast augmentation.  Implants do take time to drop and swelling to resolve.  Best of luck.