This is up to you in many ways.
Each Cores pack includes 4 individual Cores. If a Core lasts 1-2 days, one pack should last you 4-8 days. So in one month, you'll likely go through 3-4 packs of Cores.
We recommend replacing your Blank Core with a fresh new one at least every 3 days to keep everything fresh and clean.
FÜM Cores Monthly Subscriptions will always keep you stocked up on Cores. Check it out!