Hi and Welcome to YorkieTalk!!
I rotate foods, but when I buy the 6.6 pound bag of EVO or 4.4 lb Instinct etc., it lasts me a 2-3 months. My baby is about 6lbs. The only advantage to buying more at one time is that it's generally cheaper. I would definitely say that an 8lb bag will last you awhile.

You might even want to freeze half of it so it doesn't get old. It will aslo depend a little bit on what you're feeding...higher quality kibble lasts longer bc they don't eat as much each day.
Hope that helps.