You have six years from the date you sit the first SQE1 assessment, to sit SQE2. This is referred to as the ‘assessment window’. Once the six year window passes, then if you have not passed SQE2, then you will need to resit SQE1. From the point that you resit SQE1, a fresh six year window starts.
For example, if you sit your first SQE1 assessment in January 2024, and subsequently pass SQE1, then you will have until January 2030 to pass SQE2. If you do not pass SQE2 within the six year assessment window, then you will need to start again.
If you start the SQE1 assessments and fail SQE1, and have to retake, then the six year clock does not restart.
Once you pass both SQE1 and SQE2 within an assessment window, then you have passed the SQE, and you are through. After that, it does not expire.