As you are following Sprint methodology for software development, it would be very difficult to execute all the Regression test cases within 2 / 3 / 4 weeks of Sprint.
I would suggest maintaining good traceability matrix, covering Requirements -> Code -> Test cases, and based on the impact to the code/module, you can plan for focused regression testing around the impacted areas to certify the build.
You can also plan to have dedicated Regression sprint, may be after 4th or 5th Sprint (Testing Sprint, it may not be equal to the duration of your sprint, may be a week Sprint) where all the Scrum members participate including Product owners, Designers, Developers etc. which will help you to identify regression defects.
Let me know if you have more questions. Thanks.