The phrase "certain period" refers to a specific length of time. It means that something will happen or be valid for a set period, but the exact length of time is not specified. Full definition
For example, they may offer 5 % cash back on only purchases at certain types of retailers and only for certain periods of time. (magnifymoney.com)
When shopping for mortgages, car loans, leases, as well as student loans inquiries (pulls) reduce the scores differently within certain periods of time called windows. (northshoreadvisory.com)
Meanwhile, the provision that remaining debt can be forgiven after certain periods of time generates half of the program cost, the report found. (usstudentloancenter.org)