As a new NCO, when should my first NCOER be?

Please. You didn't "pin on your 5". You were promoted to Sergeant on 1 AUG 2015. E-5 is a pay grade. Sergeant is your rank, and a symbol of service to those you lead.

Congratulations on becoming a Noncommissioned Officer! Learn the NCO Creed and do your best to live it every day. Generally your NCOER should be on the anniversary of your date of promotion. The date can change based on a number of variables. You need to find out who your first line rater is going to be and have them to outline your expected responsibilities in your NCOER Support form. That way you have defined goals to shoot for. Every 90 days you are supposed to have an azimuth check with your rater to ensure you're tracking on your goals. Don't be a jerk but bring it up tactfully if it does not happen. If it gets to six months and you haven't had this counseling session, be more insistent this time. Keep note in a green book or electronically of things you have done to meet your goals. Also keep track of your APFT & AWQ, any classes you attend military or civilian. Also Any awards you may receive, medals, certificates, General Officer notes or coins. When it gets close to your anniversary then provide your list of the above to your rater and tell them you are looking forward to your review. Remember, nobody will look out for you career any better than you yourself. Becoming a NCO was one of the proudest days of my military career. Hope this helps, good luck.

Standard is due one year from date of rank (on or after 31-JUL-16 in your case)
Unless, there is a change in duty station and that should cover from the date of rank thru the last date in the unit.
Also, another reason to receive your first under a year is if you have a change of rater (90 days after that change).

You need to find out the rating scheme and if the person who is your rater is going to leave or is promoted, be sure to get an NCOER done by them if they are no longer your rater