Each cash-back credit card has limits on how much bonus cash you can earn, but many allow unlimited earnings on the lower 1% category purchases.
On higher percentage purchases, card issuers may limit the number of purchases that can receive 5%, 3%, or even 2%.
For example, the Amex Blue Cash Preferred card offers 6% on purchases at grocery stores but limits the amount of rewards-eligible purchases to $6,000 annually.