I try to keep 2 years powder and primers.
I shoot 7,000 9mm a year so 4 kg (8 lb) power and 15,000 primers.
I have about 6 months projectiles as they are usually available and the higher price makes me hesitate buying larger quantities.
I top up when I go under 2 years but will wait for up to 6 months if the price is not reasonable. After 6 months I get a bit nervous and top up anyway.
I purchase 2 kg (4 lb) powder bottles and 5,000 primers at a time.